They may well cross breed, which can be a bad thing because A you then don't have pure line offspring and things get confusing down the track if somebody buys a mixed breed offspring thinking it is pure, this is happening in chiclids and it's making it hell to decipher which fish the parents are/ where. And B often cross bred juvi's end up being bigger than either parent, not so great if one of the species you have is on the limit for what can easily be kept in most aquariums.And reason C the offsrping often wont show the best colour traits of either parent.
Sorry to be a downer.