You'd be smart to seal both the plaster and drywall with a good primer (oil base will work best or if you want to spend $50 a gallon the Benny Moore Calcimine Recoater on the plaster) before you start mudding. If you have scraped down to the drywall paper and it's peeling and loose you need to get the loose stuff off so it doesn't peel when you paint it. If you seal it with primer it helps and also you won't get all that crap in your mud as you skim.
The existing plaster most likely has been painted with a paint containing calcimine at some point back in the day which can prevent the mud from bonding and create issues when you go to paint. Whenever we do skimming over plaster we always use the Benjamin Moore Calcimine Recoater before we start mudding to ensure the mud will bond. You can do 100 jobs and be fine but there's always a few that you will get burned on so we don't take the risk of not sealing the old plaster first. Also don't use that plaster of paris crap, use joint compound. For a newbie like yourself the blue lid lightweight mud would be better than that crappy all purpose green lid stuff.
GL.
The existing plaster most likely has been painted with a paint containing calcimine at some point back in the day which can prevent the mud from bonding and create issues when you go to paint. Whenever we do skimming over plaster we always use the Benjamin Moore Calcimine Recoater before we start mudding to ensure the mud will bond. You can do 100 jobs and be fine but there's always a few that you will get burned on so we don't take the risk of not sealing the old plaster first. Also don't use that plaster of paris crap, use joint compound. For a newbie like yourself the blue lid lightweight mud would be better than that crappy all purpose green lid stuff.
GL.