Welcome to the forums!
if a dragon removes the followers of a player from a city, what happens to the city if it hasn't been scored(or a cloister)? Can any of the other players place a follower to that city or cloister?
The city counts as unoccupied, and anyone can place a follower in it ('Yes' to the second question).
And what if the city is completed by a princess tile, and there are no followers left in the city? Does it stay that way and no one scores points for it?
Again, 'Yes'. If any city is unoccupied at the moment it is completed, it scores no points - unless you parachute a meeple in from Carcassonne City (using
The Count of Carcassonne expansion).