Re: Gym Leader Applications
Dark/Poison doesn't have a double weakness. In fact, Poison is resistant to Fighting and Bug, which beats Dark, and Dark is resistant to Psychic, which beats Poison. So you have strong types, I don't think you need to limit Pokemon.
(We actually tried to do this with all the gyms but guess I managed to overlook a lot oops! The idea was to have the two types covering each others main weaknesses.)