Shihong-Yes! That sounds like the perfect use of HIM, exactly the kind of thing HE would do when the opportunity presented itself. A villain to heroine and fellow evil-doer alike. HE is not a thug with a death ray but a cunning trickster who can do and be just about anything and delights in suffering more than jewels and cash. Any misery except HIS own will do.
?-I think the boys would be beyond being affected by little pecks on the cheek by now and the girls would probably know better than to try it anyway. Also, the two I'm playing will have learned how not to grow when kissed because they don't hate the idea anymore. Brick can keep it if he wants it, though. Maybe it makes him stronger but not bigger. Or if you'd rather, it does.
Wolfy-They can also use sonics and can't be damaged by lava or acid. May be forgetting one or two, so if you notice something I missed, let us all know. Most of the powers seem to be variations and other uses for the powers you listed. For example, speed+use of energy lets call it, makes most of their special dual and triple techs. To allow them any power they want would make for long, drawn out and possibly frustrating fights, but if you use the existing powers cleverly, it should be more fun. Remember, though, they would also have had time to develop new variations on powers. It wouldn't seem too weird for Blossom to have figured out how to breathe liquid water by doing fire and ice together.
I would make Brick able to do something similar but in opposition to Blossom's powers. Something with dirt (which is not only a boyish thing [but Buttercup likes it too!]but also puts out fire) or something. Pick one for him you like, since I don't think they gave him one on the show. But, to add to Shihong's post, it shouldn't make fighting too one-sided. Instant exploding head is a bit unfair.