[Arkcon]

You kinda have the goal down. Just surviving while everyone outstrips you in technology and numbers is the challange.

Pick one race to be your ally, and let them colonize around you. Ignore diplomacy with everyone else, diplomats ... hah ... who needs their pithy little trade goods anyway,
"Lets trade research resources and advance both our races"
"Yeah, umkay, how 'bout plan B. You research it, and build it, and then I'll just take it"

Let them declare war, that way you can get ships to capture. Peace is death to pirates. Put a spaceyard base and scavanger ship at the warp points. Take 'em as they arrive.

Definitely capture colony ships. They have no defences or weapons. Use these to form colonies. Or research troops, and capture planets.

Remember, if they glass your home world, as long as you have the resources, your ships won't die. And they can't intel you if you have no planets. Build a pirate hoard somewhere no one will find it.