You're trying to make it sound like this idea is infallible and flawless. Sorry to disappoint you, but it's not. It must feel frustrating to see your expectations crumble, though.
Regardless, your suggestion will not allow more unaffiliated players to join big battles but rather slow the overall progress in Strat down, because factions will still not hire inexperienced, unaffiliated and unknown players. Factions will simply order their best fighters to join as entourage in campaigns, disabling them from trading, and not to mention almost completely eliminate cross-Atlantic mercing.
25 fighters can be whoever you want. The big factions have over 60 members, including 45+ active members.
Now add every allied clan, equally big. 25 fighters that you can trust, 25 fighters that are not guaranteed to be either contactable or competent enough to follow instructions or simply don't care. I think you'll find that those 25 slots are gonna vanish faster than a virgin in a back alley in Bronx and the rest of those 25 slots are going to be wasted simply because morons are a liability to the outcome of the battle.
One of the most ill-conceived ideas on improving Strat so far.