Sooner or later the RTC will try, whether failing or succeeding, especially if you got a female RTC that grow bigger and faster IME. If you get a male, it is less of a chance but still a risk I personally would not take, even though a 30" koi of normal proportions should be pretty bulky in circumference.
If you could remove the RTC or the koi later if problems arise, I think it might be tried with close supervision.
The point being, if a koi fits in an RTC mouth even partially, head first or tail first, it would be in danger.
You dont unfortunately, only becomes more apparent with age and size. see some of the previous threads on thebiggerthebetters fish.
That said, my 18" red tail is in with some 8" fish currently but only a couple are Not catfish themselves and hence up and about at night. For those that are not catfish it is no doubt a worrying and stressful time at night, albeit my cats are generally well fed. Even if your red tail didn't eat the koi, it would be stressing them out.
you might try other smaller catfish although I am guessing you want something that comes to interact with its owner the same as the koi do and is quite visible?