As for Grune and Pummra being unrecognizable as Thunderians. I now have a theory on this.
Grune's betrayal would seem like such a huge event in Thunderian history, that I find it unusual that none of the adult cats would be aware of it. Which means Grune's betrayal was covered up. His transgressions against the code and what it stood for were so great, he was crossed out of Thunderian history---he was exiled to space.
Now, he had a more cat like facial structure than what we're used to--and since Jaga was around there had to be a mix of these traits in the people. Because of what Grune looked like, there may have been extreme discrimination against the more cat like appearing Thunderians. In short, I strongly believe at this point that they were exiled to some sort of forbidden land after Grune's betrayal. Racist, yes, but possible. Not every society is perfect, Thunderians included.
If more cat like looking Thunderians were confined to a specific area with all outside interactions cut off, it's possible that in time they developed their own culture and traditions outside of mainstream Thundera. This took place over the course of centuries, any Thunderians with these features fazed out of normal Thunderian society.
This would explain why Cheetara, Tygra, and Panthro didn't recognize Pummra as a Thunderian, or Grune for that matter. They'd never seen Thunderians that looked like them before.