Why not just use the highest ranked player of the squad to determine the lobby the team joins? For your example, the squad with three masters and one bronze would be considered a Master squad and be placed accordingly. If there was a squad with one gold and three bronze, the squad would be placed in a gold level lobby. It would be a detriment to have lower ranked players in your squad, but that's up to the squad. Again, let's everyone play with who they want but you don't have squads trying to help boost a lower ranked teammate because the lobby they're in is more skilled.
I'm curious if that was considered, seems too obvious to miss so I'm wondering why you decided to place the restriction rather than simply use the highest rank within the squad.
The player pool at any time for a game like this needs to be pretty massive to actually separate out lobbies in the way you describe. China could do it, nowhere else really could to such precision or anything close.
Something similar to your suggestion that we have also discussed is to consider everyone in the premade group to be the same rank as the highest member for purposes of calculating RP changes. This also has its pros and cons, but would continue to limit the ability to be boosted.