Is there anyway to have more than 4 squads in the single player campaign?
Yes it is, I've been experimenting with it but the only thing I can come up with that is remotely stable is to use the script from the last mission to force the remaining two squads to be dropped near your initial drop pod a few seconds after it lands.
However this causes the game to crash if you play a mission with this additional script and you have not unlocked all your squads.
I would believe a better working mod is most likely out there or will be there soon.