You're going to be "fine" if you have one of two things happen:
1) Your mandarin is one of the rare ones that eats frozen food, -or-
2) You are willing to buy pods for him to eat and keep them well stocked in your tank.
The reason LFS's tell you things like this would be "fine" is that they want to make a sale. It's not "fine" to put a mandarin in a small tank that doesn't have the diet that they're accustomed to, no mater how cool they look.
Some are known to eat frozen mysid shrimp, and others have been coaxed to eating the fish eggs they put on sushi... flying fish roe. You might be able to find a supply of those.