The fire eel will end up in the RTC’s stomach. They can grow quite quickly, and can reach up to 40”, although their full grown size ranges from 24-40”. However, they remain slender so the RTC will easily be able to eat it if similar in size or larger.
risky, if you want tankmates for your gulper they usually ignore small community fish as adults such as endless guppies,zebra danios,cherry shrimp,tetras etc