@dark light As this RP is mostly writer-led, I think you're free to have your character ride any Pokemon they want. Common sense is the unofficial boundary... I don't think it's really necessary to come up with a definite list, or it would only make sense to do so for all Pokemon that can use all the HM moves and whatnot (sizewise, i mean. Because zubat can technically learn fly...)
I say we just let sleeping dogs lie and call people out for the unreasonable stuff, like riding Caterpies and whatnot. How about it?
CATERPIE SURFING