Go for it! A diplomat would be a really cool addition to the game, especially if he/she is some balls-to-the-wall manipulator. HOWEVER: My warning to the players who do not choose military roles is that the game will be focused on a military situation, so be prepared to be creative or have the chance of getting sidelined. This will hopefully be remedied by my GMing and the other players working to give everyone a chance in the spotlight, and the way the fleet will be led will give everyone a voice, but just be aware.