personally I really like my platy,guppies, and mollies. You could go with a decent assortment of any of those in that size tank and watch them reproduce. or get all males or females if you don't want too. a single trio 1m/2f would fill up your tank in a matter of months with new fish. and they can be kept with other types as well.
for reference of my tanks.
guppies,ghostshrimp,clown plecos.
gold dust mollies, bumblebee catfish, borneo suckers
tiger platy, cardinal tetras, L204 ( type of miniature pleco), cherry red shrimp, pygmy corys, black kulie loach.
Those are 3 seperate tanks the first 2 are 10 gallons and the last is a 20H. hopefully you get some good ideas. and Don't get suckered into buying anything until your ready! them lfs workers can be as bad as car salesmen!