plastic padding gelocoat filler works quite well, but only in white. Its not a perfect match but fills very well, cures very quickly and sticks to almost anything. Other solution would be gelcoat mixed with cabosil or fumed silica.
For both. If it is just a screw hole, a countersink drill will provide the required chamfer, then clean with acetone before filling. ECS sell a gelcoat filler too..
If its just gelcoat or gelcoat mixed with silica, it will need something to seal it from air as Pikey suggests. Baking paper works well.
I can make gelcoat stick to epoxy, but it is tricky. Not ideal for a beginner. If boat is painted, just epoxy then paint.
Anyone tried this yet? http://www.magicezy.com/