The 300 loop is closest to the dog park if you plan on using that (currently closed for renovations). As mentioned, some of the preferred sites are tigh compared to other categories.
I see you have already booked a peferred. We like the premium loops (800 & 900) for generous size and the great dog walking area that runs along the outer side. Have your dogs camped with you before? Please be sure they are accustomed to spending long periods of time alone in your camper. Barking and howling dogs are not welcome neighbors. If you are not sure of their comfort level, and youir willingness to get back to provide them breaks you may wish to consider day care at Best Friends Pet Care.
As far as requesting a loop, many do so, and sometimes their request is granted, sometimes not. There is an app available that has photos of each site to give you an idea of size, etc. No matter what - you'll love staying at the Fort!