Join Date: Jan 2012
Location: Cadian Battle Cruiser "Divine Wrath"
That, or simply use the allies to build on what you were already doing with your main list. For example, adding an SM Tactical squad in a Rhino/Razorback along with a Vindicator, to support a mech IG force with Leman Russ Demolishers could work also, maybe not as uber efficiently as plugging gaps like CC with TH/SS termies, but still a perfectly viable build.
Logic would dictate that you use them to plug gaps, but that doesn't stop you from using the allies to reinforce the overall capabilities of the main force (even adding redundancy of purpose. IG heavy support is great and all, but some other codices offer similar units which perform a similar function, for cheaper).
Personally, i think that if the allied contingent is larger than the primary detachment, you're doing something seriously wrong (unless you're doing a huge IG platoon with spammed Heavy Support squadrons/Vendetta spam - lots of troops and vehicles justifies the hefty cost - maybe you should be doing the allies the other way around if this is what you want to play??), so i would say ~1/3 at MOST should be dedicated to allies, so that you don't lose the hitting power of the main detachment (which is vital, otherwise you cannot dominate any phases of the game like most pure armies can - IG pwn in shooting phase just by sheer numbers of shots), and also dont suffer from low model count.