Custom parameter accepting multiple classes of the same base class

I have a base class which is inherited by a few sub-classes, and what I want to do is be able to have a parameter accept all of these sub-classes. So to keep things simple, lets assume we have BaseClass with SubClassA and SubClassB. What would I need to implement in order to be able to create a component that has a parameter that only accepts SubClassA and SubClassB?

Do I need a GH_Param for each sub-class and one for the base class? Do I need a GH_Goo for each subclass and one for the base class?

I am just starting out with GH scripting and it is not easy to wrap my head around the custom component/parameter/goo situation.