So i want to get back into the game and AM look amazing so looks like im back into the hobby. Im still kinda up to date with the game i just havent written an army list in years or used allies or formations yet. Which is my problem. How do i use both Cult Mechanicus and Skitarii?

From my inexperienced eyes cult mechanicus seem like the should they should be my primary force as they have a hq choice but few troops choices and less bodies on the table. Or would CM be s better ally with just a hq,2 troops n a heavy support choice? Im not sure about the electro guys.

Also what formations in both books are worth using?