@XDelphii @PugLove888 is correct on the potions. All of the potions work on all the sim forms, human and occult. I've used most of them with aliens, vampires and regular humans as well as with spellcasters. I don't have any mermaids atm but since they work on the other occults, they should work for them too. I haven't tried the immortality potion on any sims though. I haven't needed to since I just reset them to the beginning of their current life state by using the rejuvenation potion so they don't age up to the next life state if I don't want them to. Now there is a risk to a lot of the potions and they will backfire from time to time sometimes with amusing results and other times it's uh-oh, how do I fix this fast. For instance, when the rejuvenation potion backfires, it fills the age bar up and says the sim's birthday is the next day. Others may give you a curse and or turn you into a ghost for a while. Even the excellent potions will do it once in a while.
@XDelphii If you don't want played sims to age and die when you're not playing them, there is a way to have them not age in the gameplay options menu just in case you don't know about it. I have aging off for them so it doesn't happen in my game.