No don't call the Fort.
Call the normal Disney reservation number for rooms and ask them to add a request to your reservation for a particular loop. Do it at least 5 days before arrival because reservations download to the Fort a few days in advance and the Cast Members are good about trying to fulfill a request if they can (if there is a vacancy the day you arrive). Of course it's only a request not a guarantee.
900 is a small loop (maybe 35 sites) and is popular for the reasons I gave and one time I requested it, I didn't get it but got 1000 right next to it which was fine (see my signature). The Comfort Station is accessible to both 900 and 1000 and we were in like 1002 or 1003. The only bad thing about that was the trash truck came every morning at 500am (don't ask me how I know

) to empty the Meadow TP dumpsters. The driver every morning felt the need to shake the thing at the top of his truck lift and then drop it back down on the ground from about 2 feet. And there were two or three dumpsters.
But it's a great location. The only downside was when we went to the MK was to decide whether to wait for an internal bus or walk the half mile to the boat docks (flat). We drive to the other parks. But if we were to do Premiums again, I would request 900 for my needs.
Good luck with your choice.
Bama Ed
PS - 900 is a Premium pet loop as well. Most owners are responsible with regards to daily walks and noise. Just thought I should let you know it is pet.