Here's little guide to DIY:
1) Go to ..\Fractal Softworks\Starfarer\starfarer-core\data\world\factions.
2) Make a backup of everything in here. You will be changing core game files.
3) Open hegemony.faction in a good text editor. Notepad++ is a big favorite for this kind of stuff, generally.
4) Scroll down a bit until you come up to "fleetCompositions".
5) Look below there to find the first fleet lines, in this case for the Patrol fleet.
6) Update stuff like maxFleetPoints and lyWorthOfFuel so that these modified fleets can actually move a bit.
7) Start copying/pasting ships into the lines below the "ships":{ part to your heart's content! You can get ship ideas from other fleets.
8) Start editing the number of ships in the fleet by editing the values between the square brackets ([min#, max#]). If you change "piranha_wing":[1, 1], to "piranha_wing":[5, 10], for example, this should make at least 5 and up to 10 Piranha wings spawn within the fleet.
9) Rinse and repeat for other fleets.
10) Rinse and repeat for other faction files.
11) Fire up the game and have a look around!
Have fun!
EDIT: You can combine this with bigger battles to make it even more awesome, though you might start having serious lag in bigger battles. Here's how to do that:
1) Go to ..\Fractal Softworks\Starfarer\starfarer-core\data\config.
2) Open settings.json, once again in a good text editor.
3) Find the "battleSize" line, change it to something much higher. 500 is what I use myself.
4) Optionally, if you want to be able to zoom out further find "devMode" and change it to true. Keep in mind this comes with its quirks though, and it allows you to cheat.