Heh, with that attitude it's probably a good thing you haven't seen Quinto in Heroes.
Seriously though, actors who are worth their salt should be able to believably portray a variety of different characters. I haven't seen Pegg in anything, so I don't know if he's capable of doing so, but maybe someone else who has seen Pegg in multiple shows can chime in?
I hope I'm over reacting, but from what I've seen of him in Shawn Of The Dead, they could just have well gotten Eddie Izzard. I've heard a little about what a psycho Quinto plays in Heroes, but he at least looks kinda like Spock; Oh well, time will tell.