Of course it is your house and your rules, but as soon as you make it too unreasonable you will be left with your house and your rules, with no one but you there. You don't want to make her leave, or sneak out or anything. She is safest at home, right? You want her to want to come home from college on breaks don't you?
If her curfew is midnight, then that should be when all guest should leave as well. You can state what area of the house they should stay in, and other limits such as that. With a 10 pm curfew, they would have to start a movie at 8pm some movies even 7pm-(with Kevin Costner), to watch it-finish it. With a 10 pm curfew, it will cause unnecessary stress on everyone involved. Go to bed, she will be fine. If you can't sleep now, just wait until fall, when she is really gone.
IF you like the 18 yr old boy, and trust him, then it would be much safer for him to be at your house with your daughter. Even into the night hours. Odds are she is already into more than you know, at least keep her close.
Quick back story: My husband was raised with very strict parents, everyone out of the house by the time his father would go to bed.. no music could be played after he was home from work.. etc... Well, the result was, when my husband left for college, it was years before he even wanted to return. He would go to friends houses for holidays and would work every summer in the college town. He didn't want to be under their roof again. In fact we have 5 children, They have met his parents 6-7 times in their lifetime so far.
Too many times unreasonable parents, are lonely parents, into old age, the homes are full of them.
Just a thought, I know your situation is not like his, but you want her to WANT to come home and spend quality time with you don't you.
Just my 2 cents.. good luck