I've got a maroon clown, he had two jawfish (RIP), and a green banded goby with him in a 20.
They are really mean fish (mine bites me all the time), especially when they are hosting something. When there were a lot of other fish in the tank with mine he has kinda mean to them in the beginning, but then they all got there own little territory and everything was fine. I really wouldn't worry about him attacking another fish unless of coarse it's another clown fish or something that can't hardly swim......... clownfish are horrible swimmers.
I would put them in with the MG
clown and see how it goes. the royal gramma will probably be to fast for him to do any real damage, and the goby will get inside of the rockwork where the fish can't get to him anyways. I wouldn't worry about it. Then again you may have a super clownfish from hell who's favorite meal is royal gramma with a side of watchman goby.
If it were me I wouldn't worry about it.