The only thing that really counts is your amphipod/copepod population. All the talk about tanks no smaller than so-and-so, so much live rock, set up for so long, etc., are really just indirect ways of getting at the same thing.
Mandarins will generally not eat anything other than live amphipods, copepods, and similar benthic fauna. Yeah, about 1 in 20 will eventually learn to eat something else. Don't plan on it. There's no way to "train" them to alternate foods - some individuals just pick it up.
So how do you know if you have enough? Look at your tank at night with a flashlight. If you don't have literally swarms of amphipods and copepods running around, then you likely don't have enough. Obviously the bigger the tank the smaller the swarms need to be, but ime tanks tend to either have very high or very low populations of these critters. They breed most excellently when conditions are right for them.
Let's assume your popuation is low. What to do to get it up? First of all, remove any other major predators of these animals. For example, almost any of the small wrasses will out-compete a mandarin on this stuff. Next, you have to have at least -some- of them in the tank already. Most tanks set up with live rock will have some, but if you can't see any at all when you turn lights out, then either get some fresh rock that has them (easier said than done from an LFS) or order a det kit from Inland Aquatics.
Once you have some critters in there you only need to do one thing: feed them!
I find that your copepod population will take off if you take a small pinch of flake food and grind it as fine as you can in your fingers, then add that each night. Their reproductive cycle is about 2 weeks, so 6 or 8 weeks of this and you should be in good shape. Once the copepod populations are up there, and you know how to "grow" them, you will find your mandarin will have a long and fat life.
Other than the food issue they seem to be tough little fish. They apparently have an unpleasant smell/taste so other fishes ignore them. But, I wouldn't trust them with fishes large enough to swallow them whole. A mandarin with a miniatus is likely just expensive dinner.
One more thing. At night the mandarin will find a rock ledge to curl up on and will go to sleep. When it does this it becomes stiff and its colors fade badly. The fish looks convincingly dead. Don't believe it.
I've had a mandarin for 3 years now in a 55, fat and happy, fwiw.
fwiw, imo, ime, ymmv, etc.