On the other hand, it's really no sillier than riding wolves, so why not?
To avoid (or at least minimise) squealings of 'heresy' from
SW players, you need to call them Space Lizards or whatever (replacing 'Lizards' with something sexier: Geckoes, Dragons, Iguanas, dunno) and give them different background fluff, but they fight in a
SW-like style and so count-as
SW for all rules purposes.
Before you start spending big bucks, work out the availability of all the models you'll need. You've already mentioned carnosaurs counting-as thunderwolves (hmm, sounds a bit big?), and cold ones counting-as bikes, so what will count-as 'ordinary' wolves, and are there any other models you were planning on lizardising, e.g. Stegadons counting-as rhinos? (I'm kidding, but you get the point)