DrHappinessClass Limit 2 is a happy medium between the more restrictive community competitive and the unregulated casual.
I would take this further and insist class limit 1. This way, your team can't have 2 spies and the enemy team can't run 2 heavies-2 medics, a near undefeatable combination for gibus players.
MM is never going to be like 6s and it shouldn't be, as you alluded to. it would never gain an audience. The very restrictive class limits mean your newb teammates get to play offclasses without your team getting completely screwed.