I don't see slalom as an issue, if this side of the club grows, so should polo and vice versa. The requirement of supporting both Slalom and polo as parents is somewhat of an anomaly, the club has lots of polo kit that isn't being used, therefore there is no requirement for parents to buy new equipment, unless they play in div 2.
The main limiting factor is as you've said whether anyone wants to do it... It is a similar arguement to sprint and wwr, where the club can provide the opportunity to try it, and then if there is a desire to continue, they can go out and get all of the gear. New youngsters and adults alike just need nurturing in a friendly environment...
With this in mind, I am putting together a strategy to develop canoe polo with the aim of having a youth team competing by the end of the summer... These paddlers need to be supported to ensure that they remain in the sport...