I've seen RTC TRY to eat things way too big for them to swallow... basically enough to kill them.
Honestly, you are thinking about putting a living creature in harms way of another living creature.. I love catfish, and I love turtles.. and I would not ever think about adding those 2 together.
My friend had a 5" RTC try to eat a 12"+ black arowana not even 2 hours of having the arowana.. he couldn't eat it.. instead, it just ended up killing it.
I had a RTxTSN at about 18" who NEVER bothered any fish ever... lived in harmony with other fish, and little 4" bichirs.. then one day my 6" bichir was gone..