I have Least Killiefish, the smallest known livebearer and their bellies get big, and they have 1 baby every day or 2 for up to 20 fish (I think). that is what I read, but I have not spotted new born fry, though I think I have some new ones- all of the 20 fish I have are small in the first place so it is hard to tell.
When I was little I had sword tails, they were neat, 2 or 3 survived through adulthood. It was neat ot magically have more fish one day.
there is my 2 cents