The planet Troyius (sp?) seemed to possess a ruling royal system, if it were to become a Federation member, it could do so with it's governmental system intact.
Why would they have to become a democracy just to join?
If planet Saudi Arabia, or planet China joined the Federation, they obviously wouldn't be democracies. Up until a few centuries ago, planet Hawaii had a ruling King (in the 24th century) would they, with their royal system be excluded?
For all we know, in the 24th century only a few dozen of the Federation's 150+ members are