Re: How to use Call to Arms with orher expansions
There are 2 basic parts that CtA adds to the game, but the 2nd is only introduced in the "organized" rules. The one is the Deployment deck which sets up your basic units. The second is the Specialist cards, which then either upgrades or adds the Special units.
This second aspect is how it interacts with later expansions. Each new unit that is introduced has an equivalent Specilist card (or two). After you use the Deployment deck to set up your basic army, each person can select up to 2 Specialist cards.
On the one hand, this limits you quite a bit on what new units you can bring in (though you can house rule it to include more cards selected). But on the other, it helps keep each expansion balanced so that more recent ones do not become over powered.
You may also modify the rules to play basic scenarios but draw Specialist cards to upgrade those units as well.