1. You will have to figure out the name of the outfit and then unlock the texture. Maxis has a few meshes and textures that are hidden from regular gameplay view. Here is the link to a tutorial that walks you through the process.
HERE:2. No idea, but I would venture that it has something to do with the ability of the game to log on to the Sims Exchange and download game assets from there.
3. Mermaids are the result of meshes that are added to the game. At least thats the only ones I've ever seen.
4. I ran out of ideas for families long ago. I just make them up as I go, sometimes I get ideas from whatever book i'm reading or sometimes off of tv. Others just happen as a result of game play. I've taken townies or vactioners and created the whole family from them with add ins of sims I create. That is after the townie or vactioner sim has gone through the make-over process and been made less Maxis ugly.