O'Kauyon- the Tau of Chaos

Maybe you would have to make them slow and purposeful still. Seeing as they rape as it is, you do need to give them a side effect. That, or they are even WORSE (if its possible) at close combat

Umm, how could they be worse? Maybe if you gave them WS 1, S 1, and no close combat weapon options at all.

well, they also wouldn't be able to pick from normal chaos junk, outside of leaders, and then only stuff offered to Tzeentch

Indeed, they wouldn't just be able to use chaos marines, maybe thousand sons but a tau equivalent