Can't decide who to plug in.

Savage Zeta is actually super useful. It make his unique really come together imo. Facing him w/o the Zeta you can just debuff him and stun him permanently with shock/daze, but now everytime he get criticall hit, he'll cleanse all the debuffs on himself AND then gain all his bonuses for being hit after the crit is registered. He won't be as much use against a Nihilus lead, but he can be very useful against every team that can crit lol. He's very hard to kill with his Zeta, although I'm not sure how he works in a Zeta Maul squad as he would also gain stealth when he is crit which won't allow him to be hit as much.

Also, Dooku's Zeta really doesn't fit well into the Zeta Maul squad because part of it is granting him Critical Immunity for 1 turn when he counter attacks, but he can't be critically hit in stealth anyways.

Sid is always a great choice as it makes him far more versatile in any arena squad, and so does Savages. I'd vote either Sid or Savage personally, although I think Savage is probably more difficult to face on defense.