For me its kind of stupid. I think it all should be enabled. It would work like that:
3 premades can get 2 stack or 1 + 1 solo players
If u play 3+2 u play against other 3+2
If u play 3+1+1 u can only play against 3+1+1 etc.
same with 4+1 only against 4+1
1+1+1+1+1 should only be able to play 1+1+1+1+1 etc etc.
Potentially u can give people a choice if they want to play in 4+1 as solo or not. For me its real reason to abadon the game becasue me and my 3 good friends cant enjoy game togheter.