I am sure you mean by that modded/added formations won't work in MP right? Not that mods in general do not work in MP.
For MP, Mods are probably not going to work, although we have to test this. The role of a Corps CO depends on the number of subordinate units and the number of players. If you have players for all the slots, then your role is to give them their orders and adjust as needed. If you don't have all of the command positions taken, you can issue orders to any AI commanded unit, just as you can in SP.
-Jim
VC