Sion is really the only other ‘required’ zeta for triumvirate to work against a variety of teams.
Nightsisters will transform with either lead zeta (at lower gear levels you will probably see more early And widespread returns with the Talzin lead, though Ventress lead is generally considered to be the superior PVP defensive zeta at g12/relic levels, and can still do good work in P4 of the STR)
. Rampage on Ventress is probably the next most important, followed by MT unique if you are in the gear 9-11 range or Daka’s unique at g12/relic (I think those 2 are pretty interchangeable, but they both add quite a bit of strength to the team.
Then I would go back through and add the second Mt/Ventress leadership zeta.