Some species can cross breed, like convict cichlids and a jack dempsy (I've actually seen this done! Although no fry ever survived )... however alot of species out there won't cross breed...
it all depends, some species will, some won't. Like, guppies and mollies can breed but guppies and platies or swordtails can'r even though they are livebearers. And other species can't either.