You can fight them once you get magneto-plasma engines. I do it way more often than I should. But you will need good ECCM andrange on your guns and BFCs. You can get viable warships on the 30kt-40kt range, going at 10.000-12.000km/s, but you will need large quantities of galicite, and you will need to make the engines on factories if you want the ships anytime soon. If you can be slightly faster than them, and have a decent chance of hitting them from 300.000 km, then any reasonable particle beam should give you the dps to come out on top with a 50%. You have tostart racing towards that from day one, expand your economy as fast as possible, have good military academies and tactical bonuses for your ships. But you will need a little luck with minerals. With 2 billion people, you sorta kinda can in the time frame you had, if all your starting tech is research speed, and the economy does good. Its a fun hell.