some kind of african cichlids. I find them a lot more interesting and fun to watch than convicts. you could probably fit about 4 full grown africans in that tank, with frequent water changes, and good filtration. good luck!
just a tip i would not do convict because they are hard to sell and the population gets out of control really fast. do some research on these fish/shrimp
cherry shrimp
bristle nose places
guppies/platys/mollies
maybe african cichlids
Just breed shrimp. The types that breed the fastest are red cherry and ghost shrimp. Or else guppies, zebra danios and harlequin rasbora are good. Brittlenose plecos are good too. But I've heard that it's not hard to breed giant gouramis as well.