I started both of my dragonets in a 29g biocube which gave me a great chance to get them eating prepared foods since the tank is so small. BUT I also have an ORA mandarin which was supposedly raised on pellet food so I may have had a head start with mine. I also had a refugium and lots and lots of pods in the tank which both dragonets ate up completely while they were being weaned which if I hadn't had in the tank there is no doubt in my mind both fish would not have made it.
Generally people say a mandarin should be in a super large tank only because the bigger the tank the more live rock and therefore more pods for them to eat. I've seen recommendations that they need a minimum tank size of 10g with 50lbs of rock!! They dont need the swim room but the pods that are on the live rock. That should give you some idea of what they require.
If I had the choice to make again whether or not to get a mandarin I might not be so gung ho in going with one. It was ALOT of work feeding multiple times a day and keeping the power in the tank off during feeding and then dealing with bad water parameters because of the over feeding. If you have the time, money and patience then i don't see why you couldn't try it if your tank is established.