Final Edit: GW has apparently wised up to the non-HQ HQ trick. This blog entry has been rendered completely invalid now that the new IG FAQ has stated that priests and enginseers can't be taken as the mandatory HQ choice. I would argue that also applies to the HQ choice required to ally in guard to a primary list. Honestly, I am surprised that they left this loophole in the game for so long. I suppose it really only becomes relevant in the scenario outlined below because in a 'pure' guard list the other HQs are a lot more beneficial.
Oh well, I will leave this article up as a sign of my hubris and getting put in my place by 'The Man'. Also, it's still a little bit of a funny read!
So you want to bring some allied guard to the table, do ya?! Maybe you want some Vendetta action or some Hydra ground support to help swat pesky flyers out of the air. Maybe you just need some cheap melta infantry to deal with the plethora of land raiders that are soon to enter the game. But you really want that guard support without having to sacrifice a valuable HQ slot in your primary army. I mean what would Draigowing look like with Coteaz holding up the bridal gown train?! How do I max crisis suits if I only have one Shas'el in my list? Where's the love for the 2 lash princes that I need in all of my Chaos Marine lists?!
Well, wonder no more. The guard 'dex provides a convenient little loophole in the form of priests and enginseers... those nebulous little HQ choices that don't take up a slot on the force org:
"Priests/Enginseers do not use up any Force Organization chart selections, but are otherwise treated as separate HQ units."
So far I have found nothing in the 6e rulebook that requires an allied HQ to "use up any Force Organization chart selections" only that you must take 1 HQ choice from an allied army. Priests and enginseers are "treated as separate HQ units" and thus meet the criteria. So now you can have your plasma cake and eat it too!
In fact with the new bodyguard style rule (assuming that it can be used in close combat), the ministorum priest becomes an IC to be reckoned with. Or take that enginseer to support the inevitable allying of a battery of hydra flak cannons - keep those guns blasting away all game!
Of course, GW is likely to FAQ away this little loophole as it seems to fly in the face of their intention around the ally restrictions... but given their alacrity with FAQ turn times you should be able to get a good year out of this playstyle. Enjoy!
Edit: Alright I'll take a big erase on this blog entry. From the look of things the allied force org represents an entirely additional force org to your primary:
Assuming I am reading the above chart correctly, with an allied force you can now have a total of 3 HQ choices, 4 Elites, 8 Troops, 4 Fast Attack, and 4 Heavy Support. No need to play games with the non-HQ HQ selection from the guard codex... and here I thought I was being so clever!
In fact this makes allies even crazier. Now I can field a full complement of guard heavy support AND add in some Tau Broadsides just for grins. Or three manticores AND a squad of long fangs strapped with lascannons just because I feel like it! Or how about 9 leman russ battle tanks and a teleporting dreadknight for back-up?! Ok so that last combo alone is 1550 pts. But its still workable with a guard company command squad, an inquistor, a mounted melta vet squad, a penal legion squad, and a minimum GKSS squad with psycannon at 2000 points.