I made my first tail and it sucked so I quickly made a second one and I definitely like my second tail wayy more. I personally liked making my own fabric tail because I could use the exact fabric I wanted, make it the shape I want, etc. You can definitely buy a fabric tail and alter it. I haven't tried it but mers have certainly done it before with good results. You might want to check out Jadestone's fabric tail making topic because she extended her monofin in such a way that the extension is more floppy - but not too floppy so that it looks realistic. Here is a link to her topic: http://mernetwork.com/index/showthre...Jade-s-attempt

Another alteration that you talked about was closing the fluke. This could be done with a zipper and you can hide the zipper using say shelf liner as fringe which doubles as a way to hide the zipper (Mermaid Hannah Fraser used shelf liner for the ends on her first tail flukes. It concealed the zipper and I believe she also used velcro on the shelf liner to keep it even more together.) You could maybe look into using snaps or hooks too. My fluke is open at the bottom in my tail, but I left a lot of extra fabric on the end so that you can't see the monofin. The problem is my tail started to pull up over the monofin so I need to figure out how I want to close the end. I might go with a zipper.

The tails you linked to appear to be silicone and latex tails. A fabric tail is the best starter tail though - I haven't swam in a silicone tail before nor have I seen one in person, but from what I have read they are definitely different from fabric tails. For one thing a fabric tail is the easiest to care for. Silicone and latex tails need special care to prevent them from getting mold. There are some beautiful partial silicone tails on this forum. One example is Mermaid Momo's tail that she is working on: http://mernetwork.com/index/showthre...rid-tail/page7

The partial tails will cost more of course which is why a fabric tail is the best kind of tail to start with. Fabric tails are the cheapest, easiest to care for, fastest to make/get, fastest to put on and take off, lightest, etc. There are mers whose first tail was a silicone tail, but generally everyone starts with a fabric one. During the off season I dream about what my dream tail would look like, but when I do get in the water with my simple fabric tail I don't even care that it isn't my dream tail because I still feel magical and have lots of fun!