The main reason I am currently interested in photographing at the microscope level is to be able to capture the crystals in certain heavy patinas - red/purple cuprite, green malachite and blue azurite.
I think that would work
with my 20x microscope. I am curious if your "dedicated macro lenses, extension tubes, focusing rails and bellows" can get the equivalent or greater magnification?
question - I've not tried that yet, but with just the lenses, I could easily get to 4X magnification. Perhaps double that with basic extension tubes. After that, sufficient light between the lens and the subject would start to become an issue for most DSLR rigs.
What I can tell you is that the crystalline structures you hope
to photograph may not be evident at 20X. On my stereo scope, I need to go to 80X to really start to see that stuff. My macro setup does not do 80X, but I'm willing to experiment on the scope when the camera body adaptor gets here
I get to 80X on a stereo scope with dual 30.5mm 20X reticles and a 4X stereo objective.
I use a National Optical 405TBL-10-4 with aftermarket 20X reticles to do 80X. Companies like Amscope make generic stereo microscope reticles lenses up to around 30X or so that you can combine with objectives up to about 4X from most fixed-magnification brands. These are not expensive setups, my microscope (not including extra lenses and such) can be had
new for around $200 or so, and you can get a wide magnification range for around another $100-200 in accessory lenses.
The challenge with photographing through a stereo scope, however, is that the camera doesn't benefit from any of the magnification in the reticle, only the objective. this means the effective magnification through my scope may be limited to 4X without some jiggery-pokery, noting I've not tried it yet. The same would be true of many trinocular setups.
Using a true monocular microscope where most of the magnification is in the objective lens might be what you need if you have to go in that close?