I know that guineas take a week longer to incubate than chickens, but do some of the developmental milestones occur at the same times? Can I candle at day 10 to check for development the same as with chickens?
Yo can candle, but chances are you aren't going to see anything, unless you have a very bright candler. The guineas egg shell is very thick, and of course (normally) dark and speckled, all of which make it very hard to see into them. Don't toss any out unless they look clear though, and I would wait till at least day 18 before doing that. By then, it should be pretty obvious which are developing and which aren't.
I candle at 7, 14, and 21 days, but never throw out until at least day 14. I mark the questionable ones with a pencil each time and then if they still look questionable the next week I toss them. My "candler" is nothing more than a Mag-lite... works fine for me and my guineas. Usually around day 7 you see blood vessels, day 14 you can catch them swimming around and then day 21 is a dark blob. At least those are my observations.