So, I hacked out a first design for crawler type models. The most basic problem is, there are 9 layout types and 19 shape types. This means good coverage, but a lot of complexity ensuring coherent orthogonality (i.e., making sure that some parts of legs aren't too small). The other basic problem is technical, and it is that some layouts types are animated; what that means is I need to implement matrix animations.
Nevertheless, I think this may be the actual design I use, because coverage is definitely the most important factor. Good coverage means even if the design is somewhat complex, every work product adds very substantially to the number of possible output models. Still, yessh, 28 modelling types is quite a lot to keep straight.
I will work out the second design and see how it compares.