When I had my RTC he grew up with like, 7 other fish.
He never ever tried eating any of them, even though he could of easily.
Any other fish I tried throwing in there though, he ate.
I put two 10 inch Balas in there and he ate BOTH of them.
I put a HUGE african cichlid in there and he ate it.
Even though I alread had an Arican in there around that size.
I guess he just knew they where friends and not food.
honestly, I don't know how common that is though.