Sure!
I'm planning to have a refugium in the sump to help with zooplankton, though I'm realistic about the limited role it can probably play. Given that it's main role will be to try and provide critters for the main tank, I assume a combination of sand (not the fine oolitic stuff), live rock rubble and any pieces of live rock I don't need for the main tank is the best way to stock it. Any point in lighting it (i.e. to provide algae for pods to feed on)?
Yeah, light is good. Some people even hang Brillo pads in the water to provide more pod breeding ground. Maybe surface area is the limitation on their production? I'm probably not a good person to ask because I don't really "believe" in refugiums insofar as their role for providing a food source to the fish. Their are plenty of places inside the main display tank where copepods can get to and a mandarin can't already.
One last question, and thanks for your patience - if I weren't getting the Periclemenes sp/Thor amboinensis, would the A.leptancanthus be a suitable tank mate for the mandarin?
Maybe. A big enough A. leptacanthus might eat a Trimma I'd bet. They might even snag a small A. parvulus. All dependent on the relevant sizes of the individuals of course.
If not, I'm starting to reconsider my decision not to go for Chromis viridis, as they are a favorite of the wife's and would provide some welcome movement in the water column. In any case, I will try and see some of the A.parvulus in person - though the pictures I've seen have failed to excite me so far, I know you wouldn't recommend them if they weren't a great little fish. They'd also be a better fit with the small fish theme than the A.leptacanthus from what you say.
I'd avoid the Chromis viridis. They come in awfully beat up nowadays and most die within a matter of weeks. Besides that, the ones that do make it will end up being surprisingly large and aggressive fish within a few years if they're treated properly. If you've seen adults in big tanks you'll know what I mean. Not really a good fish for small tanks IMO, despite them being a popular nano fish.
If I want to get those shrimp, I might have to set up a nano especially for them. I have a feeling they would get lost in my main tank anyway...