Yah, don't give yourself too much, the game really has a hard time dealing with the insane value [money saved, research rate, build rate] that you get and really slows to a crawl during the end of turn even on a high end pc. Also, even though you may be able to build a lot of ships, the game makers have put in the admirals barrier to stop people from just zerging the other players with giant fleets, aka the stack of death. If you have a new or just weak admiral, you seem to be very limited to how many ships you can have in a fleet. Also, it seems there a max built ships supported number per planet per turn, and if you exceed that value the ships in queue after that just aren't created and the invoice is deleted. I would love and hope that a trainer option for maxing out your admirals becomes available sometime down the line.
It may be possible to modify the admirals to suit your needs as well. In the admirals table it has different values for reaction, evasion, battles fought and so on. I am not sure how exactly these values effect the admirals though. Might just need to change em and see what happens :P