Well, they'll certainly train you in how to handle the front desk, at the very least. How much beyond that probably depends on interest and aptitude.

You can still visit the gyms without challenging the leader, you know. Although even so, you may want to take care, as often there are other trainers there who are... eager to show off what they know of the gym's specialty.

The Fuschia Gym specializes in poison types, yes, but there is no gym dedicated to either dark or fairy types.

*someone needs to tell her about shedinja*