The reason I like Crossbreeding for Mictlan is because of their fragile units. Even with masses of slaves, your units will take an unhealthy amout of loss. So swarming your opponent is the only real option you have to remain competitive. FoD are your anti-mage power (like devils). I would reserve them only for fights you want to capitalize on killing mages, as they too are fragile.

The core of my armies late game tend to be HfH Imps and Devils, since you can summon so many of them and they do work in greater numbers with Blood Lust. The main issue is trying to keep your domain strong enough so you don't get dominioned out while at the same time producing masses of the 'single cast' blood summons. Personally when the Mod tools are released in full I'll be changing some of all the summons to double the number and cost (Thus Bind Serpent Fiend will cost 10 Slaves, and get you 2 Fiends) to counter the huge amount of mages you need to use blood nations. (This will be true for a few other spells too)