More females the better. I would suggest Xiphophorus Variatus at a local pond store. Mine breed every few weeks or so and probably produce around 40 babies.
I have used swordtails as dither fish for smallish cichlids.
As soon as I see a gravid female, I pull her, and drop her in a sump to allow her fry to not get eaten.
I am also using swords for dithers and just let them do their thing. Most of them become fish food. Out of the last spawn 8 survived and are growing out amongst the floating plants in the tank. No sump on the 90 gallon tank; yet. The placing fry in sump for grow out is a great idea. Heck you might even try using guppy grass for a refugium.
Convicts will really only allow one breeding pair in a small tank, so there isn’t much you can do if you are going for generations.
Most livebearers go at the same rate.
Inverts will probably go faster.
You might consider some of the easier to breed killifish. Would be interesting for a science experiment as you could remove the eggs from the spawning mop, incubate them on wet peat or a damp sponge and then hatch them out.