Enrollment at FM is not 1500 if you include middle school. Most likely in the 2200 range. As for school size in general, I believe there is a critical point in enrollment. I think as school get smaller (under 1000) the difficulty grows. Most other sports teams have the same size no matter how big the school. Let's say soccer has 30 girls, field hockey 30, basketball 30, lax 30, softball 30, etc. Mrching band may have 60 or so. Those numbers will be very similar in schools of all sizes due to cuts. So let's say all non XC sports and band in a school have 200-300 girls. In a school of 1000, 500 or so girls, that leaves 200-300 to XC. At FM, with 2200 (1100 HS and MS girls), you will have 800 or so girls for XC. At GO, 1500 or so. I know my numbers are 'made up', but I think they prove a point. FM has a lot of girls. GO has more, but I don't think the number at FM hurts them as much as you think. A school of 1000 or less is hurt a lot more.
But as the previous post states, Nike will not and cannot control state organizations. Will not happen. Some states have 7th and 8th graders. Their team is their team and they should be allowed to run.
And please stop comparing boys XC to girls XC. They are completely different sports. Girls sports are not quite as competitive top to bottom. Girls sports are becoming more competitive but still have a ways to go. Look at the dominance in many girls sports at the NCAA and HS level.