Speed caps are on individual brigade models not a hardcoded all brigade type thing.Art, AA, and AT have speed cap effects on units, shouldn't be grouped with SP ones.
Hmm other problems... Well you should be able to allow units (and possibly single unit models) to have specific brigade models while rest can be not allowed to have.If Gliders are grouped with Engineer, then land units with Eng allowed will also have Gliders allowed (not possible), while Paratroopers would be able to have Engineer brigade too (again, not right at all).
Who knows??Anyway, why do you want 5 free slots? What for?