you will be fine keeping him with the tsn until the tsn is about 30% larger then he'll be a snack. Even if a rtc was the same length, they have such a wide mouth he'd be like a noodle, SLURP!!! Be carefull when you mix large cats. Also probably adviseable to keep a tsn for a couple of month before you mix it with a rtc same problem they have the same body shape as a lima when young. They thicken up as adults luckily. I am raising 2 tsn and 1 rtc at the moment. I got the 2 tsn one 5 months ago, and the other 1 month ago, and the rtc a week ago. The big tsn is 13" he lives with 3 limas for the moment, the little one is just 5 1/2", and my rtc is just 2 1/2", I am affraid for my rtc, he'll get eatten if he goes in with any of my other cats for right now, but hopefully in 2 weeks he'll be big enough to go in with my smallest tsn. Hope that helps, oh, and i will be seperating my tsn from my limas within the next 2 months.