It's hard to keep a mandarin on its own, let alone keeping any competition for the little fella.
I trained a mandarin to eat prepared foods from a small jar that I left in the corner of the tank. He would even eat pellets.
The problem was that he was so hungry that I was polluting my tank with adding so much food for that single fish.
After a while the other fish figured out how to get food from the jar. They would wait for a skinny fish to go in the jar and stir up the food and then every tang, fox face and wrasse would be waiting at the mouth of the jar picking food off as it came out. The mandarin could no longer get to the jar and eventually went from fat to emaciated in a couple of weeks. It was horrible to watch.
I do know many people that keep mandarins or dragonets, and have done for years, but these are the kind of tanks where you look through the glass and there are tiny pods covering almost every inch of the glass.
I personally would hold off if I were you.
Hope this helps,
Dunc