Daemonhosts

You can do that in Apoc, but its not really in the "spirit" of the game. Army mixes in Apoc should make sense fluff wise, so if you can come up with a reason than its okie, but shouldn't be just put together for the sake of it being totally awesome. If you did use a radical inquisitor with demonhosts I would add something about the inquisitor not being to benefit from any greyknight abilities and vice versa or that if a greyknight unit is within a certain distance of the demonhost there is a chance that they will attack that unit instead.

True, but Gk first goal is the banishment of demons. In the fluff certain radical inquisitor groups will actually be denied the use of GK's mainly the ones that use demon weapons and demonhosts. Thus if they did work together they would be only based on circumstance and the radical inquisitor would have no say over the GK. GK's would probably feel that the use of demons wasn't required to secure victory and thus would wipe them out at the first instance. My post above was just a suggestion, like it was stated you can do pretty much anything you want in Apoc, but I think part of the fun is coming up with reasons why weird army mixes would exist in the first place.