You need at least one tank or semi-tank to keep IG safe because he dies if you look at him the wrong way. K2 would be good temporarily, but I'd suggest starting L3-37 to put in eventually. She's a pretty easy farm, and is an amazing tank. With a zeta she can even save each droid ally from dying once (doesn't work on probe droid when he self destructs though). That'd probably be the best route if you want to keep BB, Chop, and R2 for other teams. Droids are kind of stuck in a bad spot because the best droids have important spots in other teams, but they can still be really good with what they have.