1) Livebearers, which include Mollys, Platys and Guppys (and Swordtails).
2) 1 gallon is very small to raise a bunch of fry. I took mine out to a separate tank one day before but also put her plants in and backdrop so she felt at home. Only difference was the gravel and the filter.
3) Most fish can breed from a few months old.
40L - 1 opaline gourami
160L planted - 8 harlequin rasboras, 6 tiger barbs, 2 swordtails, 1 bristlenose pleco