Why did Boaz take 10 elders?
Malbim citing Kesuvos 7b #1: He expounded in front of them "Amoni", and not an Amonis. (He needed 10 to publicize the law.) Rashi - he knew that if a Chacham says a Halachah before it is relevant to him, we heed him. If it is after it is relevant to him, we do not heed him, for it affects him. Therefore he expounded first, because Ge'ulah and Yibum 1 applied to Tov (the closer Go'el); it did not affect Bo'az.
Malbim citing Kesuvos 7b #2: We require 10 for Birkas Chasanim 2 . He wanted them to be ready in any case (if Tov will decline to redeem, and Bo'az will redeem and marry Rus).
Above (1:12), Rashi said that Yibum did not apply, for Machlon did not have Kidushin with Rus. He means that it was proper to marry her together with Ge'ulah, like Yibum. Malbim repeatedly discusses Yibum, but also he says (4:13) that it was not true Yibum.
Kesuvos 7b questioned this. If so, he did not need specifically elders!
