Why don't you set the targets for the Graviton Beam to allow structures and then set a validator for the only buildings you want?
I can try that.
And I can't seem to give Arbiters stasis wards. I made the button for it, but every time I add a new ability for the Arbiter, it goes away from the Arbiter list and goes to the main list of units. I dont get it and the ability doesn't show up for the unit under abilities.