The issue with livebearers is obviously the ease with which they breed. What are you going to do with the fry? If you are going with the popular species (guppy, molly, platy, swordtail, endler), the fry have little to no sell-on value, because they are so popular. You may be able to give them to the LFS, but they are unlikely to give you anything for them, as they buy them ridiculously cheaply from their wholesalers.
On the other hand, the rarer species (mosquito, limia, girardinus, goodeids, etc.) are harder to move on because, generally, they aren't so colourful as the popular species, and that's what Joe Public goes for. You may have a certain degree of success with selling them on to other "serious" hobbyists, but then finding those buyers is harder.
When I first got rare livebearers, I bought 2 different species, in the hope that they would semi-predate on each other's fry. That worked too well, in that I've only had one fry survive to maturity!
If you want fry to survive, you need to have a section of the tank that is heavily planted, particularly with java moss, as the fry love to hide in there, and it provides some micro-nutrients that the fry love to eat.
Keep each species in a ration of at least 1m:2f, preferably 1m:3f, the males are terribly persistent towards the females, and if you have that ratio, it means that the females get a break, whilst he goes and pesters one of the others.
HTH.