the only thing I know about Mandarins is that they need a VERY mature tank, due to their dietary requirements. If the tank hasn't been set up for ......some say a year, then there is a risk of starvation.
A 90g is a good size for a dragonette. Now, the tank should really be mature with a large pod population if you want the dragonette to thrive. Many even with tanks this size, setup refugiums to grow pods to feed the dragonette. It's a great fish, you just have to see to its dietary needs. You also can't trust a lfs's word that it'll eat other types of food. You need to actually observe it eat. Never buy a dragonette without observing it eat.