Flexible filler (decorators caulk) applied with an applicator gun more commonly called a mastic gun. Take a look at the project on filling gaps around the house, there are tubes of caulk there (you only have to buy one even though it shows 12) and a gun. Its easy to apply and wipe over to get a good finish which you can paint easily.
No the filler will not, and must not, hold the light. We thought you meant that the holes was slightly larger AFTER the light was fitted into it. The hole should be filled first (see our project filling holes in plasterboard) and when its all gone hard the new light should be cut into it as if it were a new ceiling. The new light must be fixed solidly in place.