I will try that - but just for fun and personal education, so don't expect anything... I don't see how to use the existing animations on 4 legs without getting the weirdest results, and distributing a half-animated or poorly animated mesh is out of question.
Your absolutely right. I've seen some attempted and to be honest they didn't come out that well. Kind of like the back ends were dead and just hauled around. I'd imagine that eventually they will be realeasing something with pets for Sims 2. Possibly then the animations would be more accomadating for something like centaurs.
Nonetheless your mermen are amazing. And if anyone could make a fully functioning centaur, it would be you:)