Remodeling farmhouse and removed plaster ceiling and walls, but still keeping some plaster walls. Experience with drywall and joint compound, but not with plaster. Is joint compound suitable for use where drywall meets against a plaster wall? Both will be painted. Can joint compound be used on a plaster wall?

It shouldn't be a problem. I've done it many times. What I've always tried to do is chip out some of the finish coat on the plaster with a broad flooring chisel and then sink the drywall tape into the plaster. I think it finishes up nicer. When I do this I like to use mesh tape rather than paper tape, but both will work.
Good Luck.