Can you choose who you disguise as when disguising as a class with more than 2 players?
Sadly, no, there is no direct way using the disguise kit to choose who you disguise as; only the team and class. You should, however, be able to re-disguise yourself a few times until the name that pops up is the specific person you want.
The other option for disguising as a specific person is the Your Eternal Reward, which always disguises you as the person you backstab, at a cost of losing the normal disguise kit.