There are no special considerations. Answers to your specific questions:
1) Will Raw Power be able to view all the photos, even if some of them are optimized versions instead of original?
2) Will Raw Power request that Photos download the original version from iCloud, or does this need to be done manually in Photos before editing in Raw Power?
RAW Power will use the Photo library to download the images on demand. You do not need to use Photos. In addition, there is a “batch download” feature in the app, so you can download the images you are interested in ahead of working. You do not need to use that though.
3) What about if I’m just culling / rating photos using Raw Power? Safe to assume this can be done with the optimized image (assuming answer to #1 above is “yes”)
Yes, the optimized image is generally fine. However, that image is often an embedded JPEG, so it may either be a) small or b) different-looking from the RAW (as embeddeds can be)