OK, lemme give a bit of an update here. New stuff has been made. At releasable level I have a turret and a rotating sensor dish. I also have a bunch of cables which need to be tweaked (slightly rescaled, and split into submodels), but are near releasable. The only issue keeping them from release is that the UV space isn't being utilized to its full potential - I want to make a few more pieces prior to releasing this lot, but I've struggled for ideas. I have a sort of array of panels modelled and about half UVed, which should use up some of the remaining space.
As far as modular installations go, I'm yet to get past the initial snag of making it just modular enough, i.e. How do you design something that can be put together out of components that are small enough to produce a diverse range of station designs, but large enough to prevent people from losing sanity putting it together (or, more significantly, running into the inherent engine problems of having too many objects in mission and docking too many together)? I have started the process, but it's s tad difficult getting both inspiration and time (especially to UV and texture). One of my failed attempts turned into that station I posted on modding last week.
Z - I've always based everything around the single girder cell (10 x 10, as you say) and the 5 cel (44m long). The girder lengths are (# cells x 9)-1, but if you want to dock in a perpendicular orientation, you need to add 10m to the dockpoint separation distance to allow for a unit cell on either end. Almost everything fits together neatly, except for the black cross cube, which I made in a hurry, and so is scaled badly.
As for dockpoints on top of each other at different angles, this does work - the system is in place for the 90 degree bends on the pipes.
If you have some ideas, I'd appreciate any new pieces. If people are really keen, then once I get back from work (a couple of days from now) I can upload my WiPs, as long as there's an underdstanding that they are just that, and might be tweaked before being finalized.