Another one for ya!!!!

Do inquisitorial allies take up a slot in your force organization chart?Ie: Say I have 2 squads of rough riders and 1 hell hound. Can I also add a Seraphim squad or deep striking Grey Knights squad and still keep my 3 IG Fast Attack choices? The rules are unclear in the WH and DH codices so I figured I'd post the question on here. Thanks!
p.s I already know they cannot be taken to fulfill my compulsory HQ and 2 Troop choices
Thanks!