The principal at Glenallan is very well-regarded, even on DCUM. The likelihood of getting a COSA is hard to predict. Some people think they're easy to get because there are so many out there, yet there are many, many stories of seemingly legitimate hardships being denied. I wouldn't count on being able to get one unless you have another reason for wanting your child to attend school in Rockville. Which school corresponds with your husband's work address? That one could very well be a stinker too.
There are a lot of things to consider when looking at a COSA. Are you comfortable with your children not attending school with their neighborhood friends? Are you ready to drive out of your way for playdates?