How often can fish inbreed before there is a problem?

I have a 55 gallon aquarium. I currently have 1 female guppy (which just had 18 fry) and a male and female balloon molly. I eventually want to have probably around 10 male guppies with 20 females, and a pair of white mollies and a pair of black mollies. I want the resulting babies to finish out the population of my tank. I realize that in a confined area, there is bound to be some inbreeding. Does anyone know how long I have before there will be a problem? And once I have a large population, would it help if I gave some of the young fish away and replaced them with new fish, to increase the genetic diversity?