Given, yes, I would agree. Physical hybridization of dissimilar species would be all-but-impossible, and at this point not accepted.
Now, if a writer has some very, very good ideas about it… They would still have a lot of convincing to do.
Perhaps, in the above mentioned case of a leopard and wolf family, I would imagine a leopard-patterned wolf (unfortunate fellow, to be sure!), but no *physical* mixture. He'd either be all wolf with odd markings, or all leopard with an unfortunate grayness of fur.
Even mythological creatures (dragons, unicorns, et al) would not produce hybrids. But how would we manage chimeras, since they are not a codified single species to begin with? Would a chimeric animorph even be possible via the curse?
Note: This speaks solely of reproduction of animorphs affected by Metamor's Curse.
Those who are affected by other magics independent of Metamor are not bound by this stipulation. There have been were-birds caught by the curse, if I recall, who were affected by other magic prior to Metamor's. Chimeras and hybrids could be possible independent of the Curse, but if their base nature was human they are not immune to the curse because of some other magic upon them. If they were stricken with the animorph aspect of the curse whatever subsequent magic they were affected by might be stripped away and replaced with a new form, or if AR/TG not changed at all save their age/gender. Genetic predispositions such as the above mentioned were-birds is a completely individual inheritance and would be unaffected by the curse. I believe someone wrote of a Kitsune who went through this (I would expect Kitsune or similar creatures, being genetically animal and not human, would be immune but… eh, someone else's creative license?)