Here we go! Suggestions for a 10g breeding tank:
Liverbearers (aside from Pike Livebearers). Endlers would be excellent, as would my personal favourites - hifin lyretail swordtails. ^__^
German Blue Rams
Most any Apistogramma spp
Corydoras Pygmaeus - you can keep 10 in a 10g and breed them with no problem. Same goes for other dwarf corys like C. habrosus, and C. Hastatus. 10gs could also be used for other Corydoras. Sterbai, Panda, and Aeneus come to mind. A group of 5 could be housed in a 10g with little problems
Bristlenose Plecos
Crenicichla Regani
ANY number of Tetras.. Rummynose and Cardinals if you want to have a little bit of a challenge
Celestial Pearl Danios (old name Galaxy Rasboras) are in high demand if you breed them and can't house all the fry.
Pretty sure there's many more I've missed.