Am I being overprotective?
Is your son interested in dating? If he is, this is most unusual for a child his age, and it might be to please his dad. Another thought: Would you let your son go with any child, male or female, without an adult present? If you are the custodial parent you set the rules for dating and everything else. I know you want your son's father to be part of this so here are the facts: Most preteens (and even a good number of high-school students) don't date. They are most comfortable in group activities. Most of them are working hard at figuring out just how to talk to the other gender!
Many middle-school children say they are "going with" someone when they like each other and have had a friend act as a go-between and tell the other. This "going together" normally changes from day to day as they break up and "go with" another person.
As for the aggressive girl, again, you set the rules. No family needs to tolerate a child's calls, notes, or visits that disrupt family time or go against family rules. If she continues, a firm call to her parents stating your position might help.