Actually, I manage the Wiki primarily, though it's mostly managed through the linked Discord. Anyway, let me deal with your points individually. * What folder structure should I use on the external SD card? Where do ROMs go? Where do apps go? Etc...
You can structure the external SD however you want, as long as it's formatted FAT32. Apps are covered as part of how you install OPK files. * Which emulators support/require ROMs to be Zipped?
That's up to the individual emulator authors, you'd have to check with them. We're trying to get a central repository together but it's in the early stages. * What size SD card should I use for the internal SD? While finding anything below 32GB is getting tough these days, is there any benefit to a >32GB internal SD if you already have a fairly large external SD installed?
No benefit at all. I don't recommend going beyond 64 GB internal cards, and 32 GB is ideal if you can find some. The larger your internal SD card, the slower it will perform, but I can understand wanting to keep some ROMs internally and leaving the external slot free (for example, for a gift device).
And thinking ahead for when I finally get my unit...* What are the preferred emulators for each platform? Pro/cons of the available emulators?
Still shaking itself out actually, there's a lot of forking and diverging dev paths still going on. * For purposes of backing up files, where are some of the files located on the internal SD that I should back up? Save states? Emulator settings? ROM battery saves?
That actually IS mentioned in the Wiki. * I've made handheld-focused themes for RetroPie/EmulationStation handhelds (the GBZ35 & Freeplay themes) and would be interested in making UI themes for the RG-350. How would I go about learning the ropes? Is there documentation anywhere?
Join the Discord, there's some people working on skins there, and a lot of info available. And we definitely need some new themes and skins. * Is there a list of optional downloadable apps I can install to the RG-350? Instructions for installing these apps?
Not yet. Another work in progress. Sorry.