I don't have any personal knowledge about the items you linked, but I read quite a few photo chemical samples in my younger days - beyond 20 years ago. We used info from Kodak for our guidelines. Similar to what Kodak's motion picture group has available here (if you haven't already seen it): http://motion.kodak.com/motion/uploa...3_ulm191-2.pdf
I think you'd be fine with a single junction reference. Double junctions are mainly for special cases where something you're doing will cause interference. We typically used separate electrodes - this lets you rotate out the pH electrodes. And if something goes bad, you only replace that electrode.
I never personally used gel electrodes; they were either frit or anular (sleeve) junctions. A frit junction is harder to troubleshoot for clogging; with sleeve junctions, you can just open them up to drain, then refill. So if there are any clogging issues, a sleeve junction is good. Note that we were testing replenished and regenerated solutions; if you only test scratch-mixed B&W developers, I suspect that these are less problematic.
Whatever you use, you'll probably get a little faster response by gently stirring the sample while reading. It should be a constant speed, so a small magnetic stirrer works great. Calibrating (with buffers) should be done the same way.