Mollys...Before I separated the males and females, the females would regularly drop litters of 40+ fry. The problem was I had 4 females. I've never seen so many baby fish in my life! I don't think I would ever breed them on purpose unless I had a reliable fish to feed the culls to.
Endler's or sailfin mollies, IME neither one eat their fry, mollies grow larger so less fish to fill the tank. I put a couple dozen endler's in a 40 tall last year. After a few months the tank was looking crowded, then I picked up a baby Carapo knife fish. The Carapo thinned them out much faster than they had filled the tank.
I set up a large clear plastic tub on my back patio to keep some extra plants. It's now full of guppy grass and milfoil. I threw in 4 or 5 mosquitofish about 3 weeks ago. There are now too many to count.
Any common livebearer is going to be prolific. Maybe growth rates are a consideration?
If I go forward with this, I think I'd go with Platys. I'd start with 4-5 fat females & 2-3 males in a bare breeder type 40 with a lot of Hornwort. I'd be doing a dry start on the 125. By time the 125 was ready for water I should have a lot of baby Platys.