Something like...
Brackets:
Very Easy Chickens \ Very Easy Circuit, equal Player:AI teamsize
Easy Chickens \ Easy Circuit, equal Player:AI teamsize
Medium Chickens \ Medium Circuit, equal Player:AI teamsize
Hard Chickens \ Hard Circuit, equal Player:AI teamsize
Suicidal Chickens \ Hard Circuit, AI team size = 1.5*[human team size] (rounding up)
2x Suicidal Chickens \ Hard Circuit, AI team size = 2*[human team size]
Where each bracket has a range of 200 elo, with lowest bracket elo floor goes down to 0, highest bracket avg elo ceiling goes up to 9000
Elo used to decide BRACKET is avg of the player team's casual elo.
Defining sensible elo range values for each bracket probably requires a look at the playerbases elo distribution.
Really there should be a way for the players to decide which form of COOP they are doing (chicken or compstomp), or people will be pushed to restart/drop games when they get the wrong type.