To make it unselectable (which also removes health bars), you can create an ability based of Chaos which transforms the unit into the unit that you want.
Add Locust through triggers and then add that chaos ability.
Make sure that the unit is NOT the same as the unit in the chaos ability because if they are the same, the effect will not occur.
When the unit is changed, it will remove locust but still remains locust classified.
So you have an unselectable, targetable unit.
In this example map, I create unselectable riflemans which are still affected by collision and are still targetable.
EDIT:
Ow yes, if you want to remove collision, add the ability "Ghost (visible)" to the unit and turn off collision in triggers.