Hello lb and welcome to cruise-chat;
first, congratulations on the graduations; what a wonderful way to celebrate.
I could only find two ships in lines that I am familiar with, and neither fit all your criteria. Both have July sailings and both are under 800 pax, but neither is specifically a "Greek Island" cruise: Holland America's Prinsendam and Princess' Royal Princess.
I think you will have to look to Regent Seven Seas, Seabourn, or SeaDream to find the cruise you want.
Best of luck, and again, congratulations.