Hi Todd
Welcome aboard, the GB only starts next year and is open to everyone. Not sure if we've implemented an end date yet.
Officially there aren't any USMC superhornet squadrons yet. They're all USN (as you've noticed) and the Navy seems to be focusing on migrating the F-14 squadrons to superhornets first. Whether or when the USMC gets any anyones guess.
You will have to "steal" some C/D decals for your build, what-ifs are allowed so knock yourself out. However, be warned the superhornet is substantially larger than a C or D plane. The vertical stabilisers might be larger(not sure) which might cause some problems with decals for Cs or Ds.
I would assume that TwoBobs or SuperScale have various options available.