I'm gonna suggest Harmony/Lockhart Legion/Silence/Kitten Crusaders as a guild to join, your best off checking out the
Guild List.
If your more into a guild that's still in the early stages looking to start up and build members, then I suggest Crimson Knights and Defying Gravity~ Though CK does have RP, it's not mandatory and is just for fun.
I would suggest my guild since we fit all the categories but I doubt we fit your member count preference, however we do have a tight knit community still.