I personally wouldn't even risk a 3-4" royal in there. I had a 1.5" royal in a 15g on his own, and it meant water changes every other day, and there was tonnes of mess just from him. It was managable at his size, but if he'd have been 3-4", it would have meant twice daily water changes I'd think - daily gravel vacs, and still one very unhappy little plec. Any other baby plec you'd probably get away with it short term, but not a royal - they make much more mess than most plecs due to them eating lots of wood.