Royal Gramma does not bother anything except defended his house. If you have large enught tank where other fish leave the RG alone, then there is no problem.
I keep a gramma in a tank with a pair of scooter blennies. They are pretty similar in habits to the mandrins. The don't pay each other any mind at all. The gramma is about the most peacful fish in my tank. They don't bother anyone.
Ive had both in a 29g, and they both completely ignored each other.
In fact I would say a mandarin ignores anything larger than its diet staple, which is bugs one millimeter or smaller, which includes all of my fish, and me as well. Its as if he didnt see them.
I have 2 gramma's in my 50, together with a blue mandarin and a variety of other fish. No problem at all, I have the mandarin in this tank for about 1,5 years now..