Animal inbreeding happens, especially in smaller, isolated populations. However, there are centuries of inbreeding for most pet animals. White lab mice, for example, might as well all be the same mouse, for the amount of genetic variation there is. This makes them the ideal test subject, as you have a homogenous population to test on. However, this level of inbreeding among other animals, dogs for example, make for a lot of problematic pets, originally bred for one purpose, but now in the general pet field.
German shepherds, for example. Bred to be working dogs who, more often than not, died as soon as they were no longer useful as workers and had passed on their genes (if they were good dogs). Now, as a pet you have enormous vet bills as they get old because of all the hip dysplasia problems that are a genetic flaw of the breed due to the inbreeding to get the breed in the first place.
This goes all over the place. Greyhounds have terrible neuroses, exotic cat breeds are likely to be mean or sickly, etc. There are a few examples of purebred animals that one could argue the benefits gained outweigh the problems created, of course, but they also tend to be the animals that live short lives. Cattle breeds, for example. No one cares if a Jersey cow is prone to back problems late in life. They're just used for dairy and calfs until they're too old, then its off to the dogfood factory.
My original point, believe it or not, was that many livebearers were all the same species before we started mucking around in the genomes to create sunset platies, swordtails, veratus, etc, etc, etc. If nature sees fit to allow them to interbreed, then good on them. If their offspring is viable, then chances are they're mostly the same species anyway. If it isn't, then there's nothing to worry about. Its not like a guppy and a platy are going to create some horrible, eight mouthed abomination that's living in daily pain.
