Who becomes a mermaid is pretty much random from what I have read. At least one or more will show up later. I was just playing in MF and the male Ivy, don't remember his first name was a mermaid, he collapsed due to dehydration according the popup. One of the other guests at the party threw a bucket of water on him and he was fine after that. Although I don't remember exactly who, I think it was one of the Singh family. There were a couple of others, but didn't pay attention.
It is the same aspect in other towns from a different angle; in Riverview I had a created sim become RI with George Dean. Four days into the game (sim days) he comes over to visit and he was a fairy. There were a couple of others who were fairies, and two who were vampires but one of the Shallow was a werewolf and kept picking fights with another werewolf or the vampire if they came around her around.
Riordan M (?) in DV was a elven when the game started but several days later he was a mermaid. My sim was romancing with him and one day he was Elven and the next day he was also a mermaid. One of the O'Shea twins was a vampire in a game, but in another game the other twin was the vampire.
In my game in IP Honey Hamanda is a fairy as was one of the cops, not a NPC, but don't remember his name. Will have to check and see who is what from SN in the game.
Basically, it is luck of the draw aspect. You will never know who will become what in all the towns you have if you have one or both of these Expansion packs.
