Those fins are so awesome great work, I'm glad I'm not the only one still interested in this discussion, I've been meaning to make more stuff for mermaids but I have no idea how to correctly assign bones to meshes, every time I try I fail and get frustrated and lose interest, you're lucky you can make anything you want for The Sims 3 and I hope to see more from you!
skin weighting is definitely the trickiest part of the 3D process :S and with sims it's even tougher because of all the specific stuff you have to do to make sure it's compatible with the game. i'm a maya person, not a milkshape or blender person, so milkshape's tools for assigning joints to verts are very confusing to me... it's hard to keep track of everything. i haven't had much luck with trying to make CAS items that need to be assigned to more than 2 or 3 different joints because then it just becomes too much to try to keep track of within milkshape's workspace.
anyway, i made some more things, but i actually just submitted them to the main MTS site and now they're awaiting approval. i'll link the thread here when they get approved! in the meantime here's a tiny little preview featuring the new things i made: