Remove any limits to joining a clan - if somebody wants to be in a clan, let them, no matter what character size they are.
Instead make it so that effective score of a clan can become negligible through too much concentrated power in it (instead of the paltry plus/minus 10% maximum effective score adjustment we have now, make it 90% maximum, that should take care of it).
Clan bonuses should not apply at all in the first 24h after joining however (to discourage clan-hopping more than just the clan join fees).