Before you use silicone, try rubbing the lips of the gasket with a bar of soap (regular soap like you take a bath with).
Also rub the bar of soap on the sides and front/rear edges of the centerboard itself.
How light are the winds you are going to be sailing in.
A light mast in a 7.5 m2 will have you going faster more of the time, but if you are small and not
real strong, a light mast in the 6.5 m2 will get you going.
Depends of how light the winds are.