have never used a hardsail before, should a beginner be on one?
Yes, definitely, if you can get one, because they are a lot more stable. But, avoid the cambered ones.
are there disadvantages like using a board without a dagger for beg?
A dagger/centre board is to stop the board from going sideways. The fin on a shortboard does the same thing, but only when the board is going fast enough. Beginners don't go fast enough so the fin does not work!
There is a discussion on this in the thread <shortboard or longboard for beginners?>. If you haven't read it yet, you should.