if the two fish start out at the same size, the RTC would actually get bigger faster than the gator. this is one of the few fishes i have observed/kept to have a growth rate faster than a gator. this is with most variables held constant...feeding, water changes, etc for both fishes.
the gator will eventually surpass the catfish in length, and possibly weight (many years down the road) but if you're looking at starting them off at the same time, the RTC can jump out ahead quickly and may even try to swallow the gator at some point.
either way, unless they are older than YOY (young of the year) fishes, i wouldn't try keeping both together--
--solomon
ps-- and keeping older ones together would require a very large tank or pond--