To disable group forum comments, go the admin panel/ installed plugins/ settings of the group plugin and remove wall widget. 

As to the sub-categories, this is custom plugin code modification. 

I've been searching forever for a plugin for different forum categories in groups! Sooo Needed....(and more group moderators would be beyond helpful too)  Can anyone point me in the right direction? :) 

Does this exist all ready, or is there someone who can do it for me? 

Michele,  presently our forum plugin is just a light version of any forum system. There are a lot of things still should be done for core functionality and this is what we are thinking about constantly within the small team.
I may suggest you try searching  the existing plugin in the Store. For example this one: http://www.oxwall.org/store/item/544 .

Or in case there is no existing solution the best way is to find a third-party developer at http://www.oxwall.org/market/specialists .