leelee1696: Meshes for outfits show up in the clothes section of BodyShop. Accessory meshes - like horns or wings or pre-Bon Voyage jewelry - will show up in the same section of Body shop that glasses do.
mason81: Just using the HBB skintone won't change the sim's size - you have to have an HBB outfit - and its mesh - on him to notice. And for him to show up as HBB in the shower, you'll need a showerproof skintone and the nude mesh.
keena: The meshes are likely on that page - if you download all of the files that it has listed for download, you should have everything you need. The mermaid meshes will show up under clothing, just like any other outfit you download.
All: Most meshes, unless they are included in the outfit/recolor file, will say "mesh" somewhere in the download link.
I hope this was helpful ^_^