While it might be a good idea to write down your rules, don't write them in stone. There's always the chance that adjustments will be needed. Class schedules usually change from one semester to the next, bringing new requirements that may affect your dorm mates. Your boss may ask you to start working nights. A roommate takes up yoga and needs a quiet place to practice.
While it may seem frustrating at the time, trying to resolve differences and renegotiate the rules with your roommates is great learning experience. You'll wind up using those same communication and negotiation skills throughout your life -- at work and in your personal affairs.
When working out a new agreement with your roommate, know that effective communication is honest but polite, assertive but not aggressive. Stay on topic and avoid judgments. A willingness to bend can open the door to creative solutions that might be an improvement upon your original agreement.