I'm certainly not an expert but guppies are about the smallest fish that you can keep, the males certainly don't get bigger then tetras so i'm pretty sure you'd be fine with a "school" like that.
As far as agression, my guppies chase each other around and flash at each other, but they don't really posses either the mouth, or the attitude to do any serious damage. The only problem i've ever heard about from any live bearer is exahustion, or tormenting a single female, but like i said i'm not an expert by any means.