if you mean what units you can build (not unit capacity), when I started supreme commander the original campaign was easiest to learn how the game works and what unit does what exactly because units were restricted, instead of having overwhelming number of options that idk what any does of FA campaign
but then again players that come to FAF probably aren't that new to the game
bigger problem to me is that original campaign wasn't balanced around FAF or even FA balance (everyone "teaching" you to build 3 mex and 3 pgen at start instead of factory, units having different ranges/hp/damage)
it would at least be chronological, and separating them into groups (original, FA, custom) would help tell what any of them are, ordering and possibly adding cutscenes between them that you can't otherwise see (such as faction intros, and between mission cutscenes) would also help something other than gameplay that everyone is forgetting - story, idk how many even know supcom has one