there are probably some things that can be done in the Gameplay.constants file but for the most part modding anything in general is out of the question until stardock releases the forge tools since their files are in binary form.

You can change stuff in gameplay constants file, but that doesn't really seem to do anything. My guess is you would have to change the fleet logistics research options to increase the amount of fleet supply you get. Like kcissem said, that would require the forge tools.

Also don't forget that if you increase the pop cap that the AIs will simply increase their own fleets as well. Though on the other hand from what I've seen, from the five or so games I've played, the AI seems to group together the vast majority of their fleet towards the end and hit you planet by planet in a very methodical fashion, and hardly ever has subsidiary fleets of any real size or consequence. So that might stop them rushing you with multiple huge fleets, on the other hand it won't stop them rushing you with a single vast fleet. And if you're playing a large map it won't stop all the surviving AIs hitting you with vast fleets. But then again the frantic rushing around trying to defend your territory whilst choosing your own targets of attack very carefully is part of what makes the game so fun.