The AU itself isn't bad, and you've done a good job of writing the history and making it practical. There is nothing wrong with using other's drawings (as long as credit is properly applied ofc) as a base and incorporating parts of other's designs and drawings. I have done it before myself. However, if you're going to do so, rework them completely enough to where your shading, coloring, and style is consistent enough to where it's not so blatantly obvious that you've used drawings from several artists. What I'm trying to say is, it just looks inconsistent when you have 3-4 drawings of differening levels of detail and shading level. You don't have to mind my opinion, but I'm giving it, lol.
I appreciate it.