That all sounds reasonable. 

You haven't happened to see "H20: Just Add Water?"

It's an Australian TV show about three human girls who get turned into mermaids. Whenever they get touched by water, they turn into one. (Still trying to figure out how they managed with sudden rain downpours or drinking water lol ) Anyways, they each found out they had some sort of elemental power. One could control water, one could freeze things, and one could heat up things. If you wanted, you could have some mermaids (not all of them) have a little elemental power, but nothing "all powerful" like they could take over the world with it LOL

The kiss thing works, but how about making it a little more deep than that? A merperson kiss binds them with whoever they kiss? I read a book where it did that and in this book merpeople were also able to take on human legs and live among humans if needed. (If you want it could be something merpeople used to be able to do, but have thought to lose that ability long ago, or else they just forgot how to do it, because they stopped doing it for some reason or another)

With merpeople it can really go anywhere haha There are so many different takes on the legend.