I always kind of liked the Rhino as well, so I'm looking forward to seeing him on the big screen too. I definitely wouldn't say that Paul Giamatti was the first name that popped into my head, but you just never know. I didn't know about Heath Ledger or Anne Hathaway either, and look how that turned out. Plus, I know Giamatti came out a couple of years ago saying that he wanted to play the Rhino - way before the Amazing Spider-Man movie was even a thought.
I'm ok with two villains in a movie, but any more than that can get too busy. It all depends on how it is handled I guess. Look at Nolan's Batman (which for the most part everyone loves), every movie has multiple villains: Batman Begins (Scarecrow, Ras Al Ghul), Dark Knight (Scarecrow cameo, Joker, Two Face), Dark Knight Rises (Bane, Catwoman, Talia Al Ghul, Scarecrow cameo again). Sure, it can be horrible in some cases (Spidey 3, Batman and Robin, etc.), but I think two can be handled. As crazy as it sounds, the kid in me would love to see the Spidey series slowly build up to having a Sinister Six blowout. Handle the character background/build up in the previous movies and then lead to an all-out action Sinister Six movies. Ideally, in Spidey 6, although it is unlikely they'd keep the cast for that long (or they'd be too old). Anyways, Rhino sounds good to me.