Ive had exactly this problem from the first time I got Sims 3 Island Paridise. The Lag, and the mermaid tail not showing up. I would love answers because it is a great pack.
June 2017 - last edited June 2017
Yes it works definitely but they will lose their own scale color they had if they were NPC ones. I used this way for Mia Azule and she got purple scale instead of her green one, so you need to fix her scale color to the old one one more time. The best way is give a friendly introduction while scuba diving. Then come back home, and make them as your good friend using cheat, then invite them to your home. Take them to CAS by cheat, set their age to younger age, then back to live mode. Then use 'Age Transition' cheat, and let them age up themselves, then they will get their scale and scale colors back. Then try to go swimming with them for test, they will show you beautiful scales. Especially, I love the scale color of Triton King.
I don't have lagging in Isla Paradiso because I moved all households from houseboats to the land. Also I moved 'Medina' household to other house because his house causes routing issue. Then I changed all the houseboats and Medina's home to No visitors Allowed community lot. I don't have lagging after this.
I have loaded a video to YouTube demonstrating using a mod to remove the mermaid occult then add it back to the mermaid missing it's tail and scales. This process will put the default scales onto their legs. If they are under water when you do it their tail will appear when they go up to the surface.
If you want them to have their original scale colours for the four Isla Paradiso mermaids you can take them to a mirror, or use a mod if you don't control the mermaid, and change their scale colours to the correct colours.