Girardinus falcatus. Males to 3/4" and females to 1 1/2". A colony of 40 would fit just fine in a 10 gallon with a good sponge filter.
1-5 fry are released each day for about a week every 20 - 40 days.
The males are quite amusing to watch, as their gonopodiums are fully half the length of their bodies, and they "swordfight" for territory.
Look for more than your LFS has to offer. Pick up a Baensch atlas and look under "livebearers" for so many more choices! Join an aquarium society and learn from others, or better yet, find a fish show (there are lots every year) and go to some lectures... explore your hobby!!
/Kris