Could a Druid be RPed as aligned with the Avengers of Hyjal but able to take on a flame kitten form? I'm not sure if the flame cat form is exclusive to Druids of the Flame. I've seen the Avengers using Flame Hippogryphs and flame-themed armor, but haven't seen one use the cat.
The flame cat form is for Flame Druids only, yeah. It's a corrupted form, given to them by Ragnaros, so a non-corrupted druid could not obtain it.
Dern. And I guess immolating myself isn't a good enough reason to use that sexiness of a form.
If I remember correctly, Druids of the Flame are playable under mini-CMCs. So, if you're really interested in playing with that form, then you could do a Ragnaros-aligned Druid of the Flame if you were looking at it.