Why does the Torah not specify the names of the fish?
Ba'al ha'Turim: Because they are invisible to the human eye. 1
Moshav Zekenim (10): We can say that they do not have names. Adam gave names only to the Behemos, Chayos and birds.
Perush ha'Tur ha'Aruch (which was primarily a Perush on the Ramban; printers picked out his own Chidushim, and ipublished it as [the standard] Perush ha'Tur) does not mention this. Also Tinshames (a mole, which burrows underground), Peres and Ozniyah (which are not frequent in settled areas) are not normally seen, and the Torah named them! Perhaps since the Torah named the other Sheratzim and birds, it named also what is not normally seen. (PF)
Tosfos (66b DH Kol) - perhaps this is a tradition from Adam; "Kol Asher Yikra Lo" can include fish. Alternatively, it is a tradition from Sinai..
Surely there are more Tahor species, for everything on land has a counterpart at sea, except for a weasel (Chulin 127a)! (PF)
What exactly is "Kaskeses"?
Anything that is completely attached to the body like the skin does not fall under the heading of "Kaskeses", and what Chazal mean here is that they are static and do not move around like the fins do - but they can be scraped off with a knife (See Ramban).
What is special about fins and scales?
Moshav Zekenim: The Livyasan has them. 1
Ramban, Moshav Zekenim, Shlah ha'Kadosh (17, and Hagahah): They dwell towards the top of the water and nurture from the air, which makes the body healthy. Fish without fins and scales dwell in stagnant, rotting places. Shlah - also, fins and scales are like a shield; such fish are pursued, and not pursuers.
Moshav Zekenim: It is called Livyasan - the letters spell Levai Ten (give to the nation that accompanies Hashem), and Lo Yiten - everyone in Yisrael will come and say 'give to me.' Levai Ten - everyone who clings to Torah, which was given after 50 days, to him I will give Livyasan. (I do not know how these words hint to 50 - PF.)