There is absolutely no reason to get Canderous' zeta. His ability to apply Tenacity Down and DoTs reliably is what makes him great (and the fact that he assists and attacks really frequently), so giving him Crit Dmg Up isn't necessary at all.
T3's are great with just Droids and they do improve the Ebon Hawk but not VASTLY. If you have him combined with HK, it's fantastic.
If you already have Jolee and Bastila's (I highly recommend), I'd go with those first because they're great for Raids, GA, and TW, as well as Territory Battles.