C.K.
Hi B.,
You are not overreacting, and you are not doing anything wrong, and this is not just the way it is these days. Your rules are your rules, and visitors in your house need to obey them.
At the start of the next sleepover/playdate, go over your rules with your daughter and her friends. Tell them that if they decide they don't want to follow the rules, they will be brought home, plain and simple.
If they make a mess, have them clean it up before they leave. Stand there and watch if you must.
I would not put up with some 7 year old girl insulting me to my face. She would be brought home IMMEDIATELY--better yet, I would call the parents and tell them she was waiting on the porch--and I would tell her parents what she said, and make it clear to that she was not allowed back until she changed her attitude.
Please reconsider your daughters choice in friends and who she socializes with. This does not sound like a good bunch. Children who behave this way--what kind of home life do they have? Do you really want her at their houses, either?
Good luck to you.