Do fish only breed with the same species of fish?

Yes.

Mostly. Guppies and mollies can crossbreed, but the offspring are all male and sterile. It would seem that mollies and guppies are not the same species, but they are close enough. Like horses and donkeys, the offspring are essentially mules. It can be done! See here: http://www.monsterfishkeepers.com/forums/showthread.php?t=115920 It's not worth the effort, however! The thread you posted took me to a bunch of amateur people discussing if it is possible and reporting second hand rumours of things that they have heard about. There is Nothing factual there! If you have a laboratory and can do all the required genetic work you may be able to cross the species artificially. Many things are 'possible' as in the development of the 'Parrot Cichlid' in Taiwan. But by normal (sexual in aquaria) methods it is quite impossible. Horse and Donkey and Zebra are all in the same Genus Equus and can be cross bred. The vast differences between the genus Mollienisia (which has several species) and Genus Lebistes (species reticulatus) do not allow for this. IMO The folks you have quoted are simply DREAMERS.