chickens

In a self-hosted game (One in which you have rights), !cheats, /team 1 (Depending on which team they're on, if 2 players, generally team 1).

Do that at the beginning..?

I think you can still just go on the same team as them and constantly spam /take to get their units. I don't think that was ever fixed/changed.

A few months ago Ivica made the playerchickens modoption. You can turn that on, join the team the chicken is on and start. When the first chickens spawn you will get to control them.

Yeah you just need a teams/1vs1/coop/w/e host.
You need at least 2 allyteams (you vs someone else).
Use command !setoptions playerchickens=1.
Next. You add for yourself (your team) chicken AI bot, do not fret, AI will constantly give you units.
Once balance fits your taste. (you ignore AI player in your mind)
Just do !forcestart or !start if host is AI-friendly and doesn't kick AI.
And once you are in the game. You "chicken player team" shouldn't get commanders on gamestart, but once you see roosters spawning units you'll notice that you have full control of chicken units.