There is a mod that will do something like that for you. AI Wave Survival, which is meant for playing survival on any map against the AI, can also be played human vs human.
The smaller team will spawn with the HQ, where the waves spawn at. So if you set up a game of 1 person vs 5, or 2 people vs 4, etc, the smaller team gets control of any waves that spawn.
You can also set how many units and how frequently they get reinforcements. If you somehow manage to kill the HQ, it will eliminate the team in possession of it.
Just turn off the nuke scripts, they autofire on human HQ players by mistake.