Yep, sounds abusable
We certainly can't have an alliance made of all the new joiners due to no officers, and the other option to have new players joining "random" alliances to go over the tag limit sounds so abusable as you can just leave if it's the wrong alliance and then "rejoin". Even if we increase the "loyalty bonus" and make it really clear and integrate the history of planets, it's still going to not be enough for people to not cheat