Mermaid Rules:
1. Mermaids don't like others to know about them. So they keep quiet about what they are unless they wind up in a serious relationship.
2. They can be dangerous if cornered, so don't do it.
3. they evolved right alongside humans, so they date back to the same time periods. Age them accordingly.
4. While they are long lived, be intelligent with their age. I don't want anyone to say they've got a Mermaid that's over ten thousand years old.
5. They do turn fishy when they get wet. But it takes time to fully shift. So scales are first and easily covered.
6. No guns! These guys don't need them!
7. Weaknesses will include: being away from water for too long (as in days or weeks, not hours), some form of metal to be poisonous to them, they may or may not find precious gems to be a distraction that they have a hard time breaking away from (for trapping purposes only), fatal wounds (stab anyone in the heart and they're going to have major problems), and I'll add more as they come to me.
8. While they do have magical abilities, they're all limited to water-based stuff, singing for the Sirens, and an ability to manipulate precious gems (if they're not having the 'fascination' problem).