How to limit # of forums displayed under mainpage categories, pagination, collapse ?

[ZiNgA BuRgA]

That won't help you.
This is somewhat difficult to implement.

Realistically, one would question why you want pagination.  You should probably be categorising your forums more if you have too many of them.
And clicking is far less user friendly than scrolling, so you don't gain much from that.  Note that this form of hiding also makes stuff like search more difficult to use.