A galka on a chigoe is a little excessive. Would just be wrong to see. I would say if it was ever implemented, there would have to be restrictions on which races can ride which mounts.

Galka: Dhalmel, Yovra, Wyverns (big ones).
Taru: Chigoe, Rarab, Wyvern (small ones)
Hume/Mithra/Elvaan: Everything else.