The problem is that loaches have spines on their heads. Eating a loach could seriously harm the RTC, and possibly kill him. You really need to get this fish into a proper home or take him back to the store. The situation keeps getting worse and worse for the little guy.
If the rtc is currently 3", then expect the loaches to start being eaten soon. I had a 3" rtc eat a 2" p-bass not too long ago. If it fits in the mouth, it will be eaten!