For casual mode, why not? I think to do it, I would make it so stacks can only be matched against a team with a same-sized stack. So if you are 2, with 1 + 1 + 1, you can only get matched against a 2 + 1 + 1 + 1. Would mean playing with someone else is a longer queue sometimes, but overall the cleanest solution for that unranked environment.
I think ranked stacks are also possible, but like many have said, more complex. There is indeed a way to statistically weigh team sizes within matchmaking. Think that's a good long-term goal for the PvP team, but I think getting casual mode with friends is a better place to start.
