Pretty sure RTC is not a suittable companion for any fish.
Fishes like the arapaima would be fine tank mates, if you got both at adult size, but the RTC is likely to keep growing, and growing, and growing more before your pima has a chance to even try to catch up.
If at all the case (take what I say with a grain of salt, my two favorite fishes, but never had either), then your RTC will end up with a cherished and very pricy meal one of these days.
Chitala on the other hand sounds alright, the one I had didn't grow fast enough for my liking, but I'm sure given a proper size, these two could work well together, for a while.