Yea, krylon Fusion is a fairly good plastic paint. The other options are plastic prep primer with a normal paint over top, like car paint. Either way I woulnd't expect too much from it holding up for a long time.
Depends on how much it flexes. If it's the real gummy stuff, it probably won't last long. There's plenty of masks floating around that have been painted and look good, at least when the pictures were taken. Take your time, follow the directions and don't rush it and it should be fine.