Ohh I was gonna get them but there was only one left at the store yesterday, and I've been stuck with lone fish before and it's not fun.
They only eat from the surface, live foods are your best bet, people have had success with wingless fruit flies (you can buy live cultures, they are used to feed small reptiles). Could probably get them over onto flakes. Water chemistry, it's a gamble, but basic water is your best bet. Hard to breed, they are livebearers but the young are often still borns, thought to be from nutrition deficiencies in what they eat, I think vitamin D is the one in question, so that'd be something too look into if you want babies.
Very skittish fish at first, I've heard that they can ram into glass aquariums and break their faces, though if it's gotten to a dealer's tank it can't be that stupid (well, I think at least). Uhh, that's about it. . . Males are aggressive to each other but in a 20ish tank you shouldn't have to worry about it too much.