They are finishing my install of a central A/C today (Carrier Infinity - 5 tons; 2-stage condensor; var speed A/H; one zone with return in upper hallway ceiling). Can they test it if the outside temp is 60 degrees or will that damage the condensor?

Also, they moved the return from above the stairs to the ceiling in the hallway - what should be done to seal this return so that cold air doesn't come into the house from the attic? They left the metal box that has a round opening in the back - should I use duct tape to cover? What about using insulation of some sort to tape on the back of the box? Thanks.