I've got some pretty decent photoshop skills and have been thinking about trying my hand at decensoring some art for a while, but I had a few questions.
1. Are there any unwritten rules past what's on the actual rules page that I should be aware of?
2. Is .png preferred on edits to avoid additional artifacting, or is .jpeg preferred to avoid file bloat?

Check the artist's entry to see if they have a no-edits-allowed status.
High-quality images are always preferred over their lower-quality versions, so PNG would be the go-to answer.

higher file size is better than double jpeg compression.

All guidelines are written in plain english text:
uploading_guidelines (can be found on top panel Wiki)

So general guidelines you want to look at are under Edits. We are keeping pretty high bar on edits, you can however always link these from third party hosting sites and link them in posts comments or description.

With uncensor edits, machine/AI learning alghorithms and upscale of mosaics is completely forbidden. So simply running the image trough deepcreampy or running mosaic area trough waifu2x will be deleted.
Denoising edits are not allowed either, but if the source material is so compressed that it would cause clear visual difference between properly edited and non-edited areas, it can be seen acceptable (othervice you would need to emulate or apply compression on edited area), however this should also be avoided and upscales are strictly not allowed at all.

Yes, to avoid further degradation of the artwork, edits should be saved as PNG. If it seems to bloat up, you can try to losslessly optimize the file before uploading. Lately been using Pingo because of how fast and efficient it is, just remember to use lossless quality settings at max compression.