Gemara

(Mishnah): The following join (for the Shi'ur to be Mekabel Tum'ah) -- fabric and sackcloth, sackcloth and leather, leather and matting.

R. Shimon says, they join because a common Shi'ur of Moshav applies to all of them.

Yoma 81a (Mishnah): Eating and drinking do not join (to be liable on Yom Kipur).

(Rav Chisda and Reish Lakish): Tana'im argue about this. (Our Mishnah is like R. Yehoshua);

(Mishnah - R. Yehoshua): If the Tum'ah and Shi'ur (e.g. of Nevelos or parts of a Mes) are the same, they join. If either or both are doubt, they do not join.

Rebuttal (Rav Nachman and R. Yochanan): It is even like Rabanan. Regarding Tum'ah, they hold that even things with different Shi'urim join, for all Tum'ah is one name. Regarding Yom Kipur, it depends on relief (from fasting). Eating and drinking do not combine to give relief.

Rishonim

Tosfos (Berachos 39a DH Batzar): There is a Shi'ur for a Berachah Acharonah. The Ri says that Borei Nefashos Rabos v'Chesronam is not an esteemed Berachah, therefore it does not require a Shi'ur. This is wrong. Borei Nefashos was enacted corresponding to Al ha'Gefen, which requires a Shi'ur. The Yerushalmi asked why R. Yochanan blessed after eating an olive, and answered that it was a complete Briyah (creation. It did not say that he blessed Borei Nefashos!)

Poskim

Shulchan Aruch (OC 208:9): If one mixed flour of legumes with flour of the five grains and made bread from it, if he does not eat a k'Zayis of grain in Kdei Achilas Pras (the time to eat a half loaf, i.e. the volume of three or four eggs), afterwards he blesses Al ha'Michyah. If he cooked them in a pot, he blesses Borei Nefashos.

Source: Beis Yosef (DH v'Chasuv, citing R. Yonah).

Magen Avraham (15): Since for all Isurim, one is liable only if he ate k'Zayis in Kdei Achilas Pras, the same applies to Birkas ha'Mazon. However, if so why does he bless Al ha'Michyah? We can say that regarding Al ha'Michyah, one blesses for anything with the five grains, even if it is less than k'Zayis in Kdei Achilas Pras. However, if it was cooked, why does he bless Borei Nefashos?

Gra (DH uliv'Sof): The correct text of the Shulchan Aruch should say 'afterwards he blesses Borei Nefashos', like a cooked food without k'Zayis in Kdei Achilas Pras. Every Berachah Acharonah requires k'Zayis in Kdei Achilas Pras.

Mishnah Berurah (47): The Acharonim found the Shulchan Aruch very difficult. The Gra changed the text. One who fears Shamayim should eat such bread only inside a meal (of proper bread). However, we do not protest against one who follows the Shulchan Aruch. The same applies if the bread had k'Zayis of grain in Kdei Achilas Pras, but he ate less than a k'Zayis of grain in Kdei Achilas Pras. He says afterwards Al ha'Michyah according to the Shulchan Aruch, or Borei Nefashos according to the Gra.

Mishnah Berurah (48): One can say Al ha'Michyah only if he ate k'Zayis of grain in Kdei Achilas Pras. This is whether he mixed grain flour with other flour (the grain is not so noticeable), or with honey, spices or other ingredients, and there is not a k'Zayis of grain in Kdei Achilas Pras. This applies to potato Knishes or Burekas. However, the custom is to bless Al ha'Michyah on a k'Zayis of sweet cake, even if there is not a k'Zayis of grain. Perhaps because the spices are to improve the food, they join with the food to a Shi'ur, like the Magen Avraham below (210:1). L'Chatchilah one should ensure that there is a k'Zayis of flour.

Shulchan Aruch (18): One may not include any addition in Birkas me'Ein Shalosh due to Safek, even though he does not add Hash-m's name and Malchus.

Levush (18): If one ate a fruit and he is unsure whether it is from the seven species, he may not drink wine and include Al ha'Peros with Al ha'Gefen. It is better to bless Borei Nefashos. This requires investigation.

Magen Avraham (26): If he has nothing else (that obligates Birkas me'Ein Shalosh) to include with it, it is better to bless me'Ein Shalosh, for then he is Yotzei with in any case. Regarding a drink, he should not bless at all. We are more concerned for a Berachah l'Vatalah than for consuming them without a Berachah.

Shulchan Aruch (210:1): One who eats less than a k'Zayis does not bless afterwards.

Magen Avraham (1): Keneses ha'Gedolah says that all foods join to a k'Zayis. If one ate half a k'Zayis of the seven species and half a k'Zayis of other food, he blesses afterwards Borei Nefashos. It seems that the same applies to bread and another food.

Machatzis ha'Shekel: Letter of the law, he should bless Borei Nefashos also on the seven species. Since they are important, Chachamim fixed a special Berachah for them. Since he did not eat a Shi'ur of the seven species and cannot bless the special Berachah, they are no worse than other food, and they join to a k'Zayis for Borei Nefashos. Some Poskim hold that one blesses Borei Nefashos for Chetzi Shi'ur of the seven species. Even though the Halachah does not follow them, here that there is a Shi'ur through joining, all agree that one blesses Borei Nefashos.

Beis Meir: Seemingly, Keneses ha'Gedolah means that all foods join when they have the same Berachah Acharonah! If they have different Berachos Acharonah, how do they join? Rabanan argue with R. Yehoshua only regarding Tum'ah. In any case, the Halachah follows R. Yehoshua. What is the source to join foods with different Berachos? Since we rule that one does not bless at all for less than a k'Zayis of the seven species, the same applies if he also ate half a k'Zayis of other food with it. Likewise, bread and other food should not join to a k'Zayis.

Sha'ar ha'Tziyon (2, citing Keneses ha'Gedolah): We bless Borei Nefashos due to a Sefek-Sefeka (two doubts). Perhaps the Halachah follows the Ri, and one blesses for a Chetzi Shi'ur of the seven species. And even according to those who disagree, since in all he ate a k'Zayis, he can bless Borei Nefashos.

R. Akiva Eiger: It is difficult to resolve the Magen Avraham with what he wrote above (208:26).

Kaf ha'Chayim (3): Eliyahu Rabah, Birkei Yosef, Chayei Adam, Kitzur Shulchan Aruch hold like the Magen Avraham.

Kaf ha'Chayim (4): Even if one ate half a k'Zayis of Peros (of the seven species), and half a k'Zayis of Mezonos or bread, he blesses Borei Nefashos, even though he did not eat anything on which we bless Borei Nefashos. However, for half a k'Zayis of Mezonos and half a k'Zayis of bread, one blesses Al ha'Michyah.

Yabi'a Omer (1:12): The Poskim ha'Rishonim and most Acharonim say that Borei Nefashos does not exempt one who needed to bless me'Ein Shalosh. If so, seemingly one who ate half a k'Zayis of the seven species and half a k'Zayis of meat, fish or vegetables should not bless Borei Nefashos, for it is not an all-encompassing Berachah. If he ate a full k'Zayis of seven species he may not bless Borei Nefashos. B'Di'eved it is a Safek Berachah l'Vatalah. If so, how can the seven species join for Borei Nefashos? Keneses ha'Gedolah says that some say that we do not bless afterwards at all, and some say that we bless Borei Nefashos. The latter is more reasonable, for it a Sefek-Sefeka. Even the Rosh, who says that one does not bless for a Chetzi Shi'ur of the seven species, holds that it joins with half a k'Zayis of other food.

Yabi'a Omer: Keneses ha'Gedolah said so because he holds that Borei Nefashos exempts me'Ein Shalosh. We hold that it does not, and some say that it is a Berachah l'Vatalah. If so, why did the Magen Avraham bring this opinion l'Halachah? He himself (208:26) holds that Borei Nefashos does not exempt me'Ein Shalosh! Perhaps he relies on the Sefek-Sefeka. However, this is a weak Sefek-Sefeka. The Ri is a lone opinion. The Mordechai says that the Ri was unsure. See Gilyon ha'Shas on Berachos 39a.

Note: R. Akiva Eiger says to see Tosfos 49b DH Rebbi. It seems that that entire Tosfos is in the name of the Ri, and one of his proofs that a k'Zayis is the Shi'ur for a Berachah Acharonah is the case in which R. Yochanan blessed on a k'Zayis.

Yabi'a Omer: Also the latter Safek is very poor. Since Borei Nefashos does not exempt the seven species, how can it join (unless we rely on the Poskim who say that b'Di'eved, Borei Nefashos exempts the seven species)? Further, we hold that one does not bless due to a Sefek-Sefeka, and even if there are 100 Sefekos. The Chida, Chayei Adam, Pri Megadim and others say so. Kehunas Olam (100b, citing his father) argues with Keneses ha'Gedolah, because the Rif, Rambam and Tosfos argue with the Ri, and the Rosh was unsure. [Yabi'a Omer - really, the Rosh Vadai argues with the Ri.]

Yabi'a Omer: R. Akiva Eiger and the Beis Meir questioned the Magen Avraham. Beis Yehudah (2:98) supported the Magen Avraham from 208:9. Even though one ate a k'Zayis, since there is not k'Zayis bi'Chdei Achilas Pras, the Berachah for grain goes away, and the Berachah for legumes returns. The same applies here (to join for Borei Nefashos). I disagree. There is different, for the foods were mixed. We bless Borei Nefashos, for we follow the majority. Alternatively, perhaps there one blesses Borei Nefashos only if there is a k'Zayis of legumes. When they were not mixed, one should not bless afterwards at all.