It takes pandas forever to get big. I have some I got a year and a half ago at the lps, and they are tiny like juvie fry. I thought that that was as big as they get, but I recently received breeding pandas from a private breeder and they are very normal size. So either the pandas from the fish farms in Asia are puny little things or my fish farm pandas are still growing up. Do be advised though that most pandas in the lps are force bred in fish farms in Asia that are compared with puppy mills. Therefore lps pandas have very delicate health. Nevertheless it is my understanding that pandas mature very slowly.
Pandas that are fish farm bred are sensitive to other boisterous larger fish. I would keep them in a small separate tank.