If you are creating a unit that is suitable for either West or East (like mercenaries), even if it suits West better than East (but it could pass as an East unit, if it really has to), please make it a Rebel instead. Or at least make two version of it (one west, one east).
I can't agree on that one, since it's a little bit waste of memory size.
There's an easy way to use units from one side for another side:
E.g: you want to use west units but on eastern side.
Just create an east unit with a higher rank than the west units and set his probability of presence to 0.
Now group the west units with the east unit (F2 and drag a line), and they will be fighting for the east
but looking like the wests.
In OFP there was that 12 units per group limit which again limited this method to have only 11 west units
playing as eastern in one group, but especially in ArmA, where there's not such a 12/group limit it doesn't