This is a real simple topic on something I personally would like to see. (If you have any reasons against or for that I haven't thought of please feel free to comment as I'd like to get an understand about this tiny aspect of the game, Thanks!
Before I had the chance of seeing the power of hiring a fleet at a port I'd reached a level where I was unable to hire one. Not only did this make a mission that was of the same rank as myself difficult if no friends were online to help, but trading alone without a fleet is a scary thing indeed! My suggestion is to allow, larger ships to be purchased as a fleet when your rank increases and allow higher ranks to also purchase even if its only one!
Thanks,
BlackestBeard