I have a single mandarin in my 90g and he's quite fat, for a mandarin. I have had it for ~10 months, so I know it's eating. I have probably 80+lbs of LR, with a relatively open rockscape. However, the rock I have is very porous.
I would wait for the copepods to establish themselves before attempting a mandarin. Also, the more you feed your other fish, the less likely they are to develop a taste for the copepods. I can see copepods on the glass all the time, so I know their population is stable.