well guppies or platies will breed to fill out your tank quickly
You'd be surprised at how many do survive. I had Guppies with my GBR's and female Betta, still had 5+ fry survive with each batch. Granted they did pick off a lot of the fry, they don't get them all.the fry will all get eaten...i domnt thing many woud survive
It sounds like you want lots of little schools/shoals of fish. Am I right?
Are you looking for specific colours? Behaviours? etc. Or do you just want a list of fish that stay under 3 inches?