Must-name-next-djinn-baby-"Igazor"! Ha! Love the idea! :D
I always thought of the sim mermaids as mammals. Because they can reproduce with human sims. Also, those shells have to hide something which is totally useless to a fish, right? And their tail fin looks more like that of a dolphin (mammal), than that of a shark (fish).
Even if they were fish, why would it be weird for them to eat other fish? Fish eat other fish. Humans are mammals and they eat other mammals all the time (well, I personally don´t, but I used to). If they started gnawing on each other, that would be something different.
Speaking of vegetarians, if you eat some meat by accident, it´s not enough to feel bad about it, you have to puke your guts out.