He said if we want to sit on the floor thats cool, but we need to give up our seats for people that enrolled before that person made the mistake.
...
I talk to the head of the Dept and ask him if i can bring in a chair and use a laptop and if a couple other of us in this situation could too. He says no, no exceptions, you'll have to wait six months or till another time.
What, they're going to kick you out unless you sit on the floor, no exceptions? Bring in a damn chair. It's not like the prof's going to beat you up, throw the chair out the door, and tell you to sit on the floor like a dog.
Personally, I'd take it up the admin chain. If their mistake is going to cost you money/time, you can look into legal recourse.
Are you at a regular university, or some for-profit shithole?