Ok, I think I've found out what goes on with them.  The mummies will attack orcs on their own (and I suppose madmen).  The ghouls and drybones will never initiate attacks on their own, but if they come across a Mummy in battle, they will assist it in killing the Orc(s). 
 
I'll implement it thus. 
		
		
		
		
		
		
		 |