I suspect this could be a fairly significant issue, especially in the ranked match side of things.
From what I can tell ( and this lines up with issues folk have reported about previous beta save games ) the fleet size implementation is purely client side.
One of the tools I was working on was an out of game fleet editor. While the editor has worked well, I wont be open sourcing it at this time. In essence it's trivial to create a fleet of arbitrary ships without any size restriction. While you wont be able to edit the fleet in game, it can still be selected and deployed.
Just to confirm, I was not part of the previous beta and this is not a legacy fleet.
edited to fix title