My current theory is to have picket ships, long endurance fighters that cycle to a fighter base, not so much for defense as for warning and tracking.
The second priority is point defense for a colony, so that it can't just be cheaply blown out of existence from long range. This allows me to take advantage of the fact that storing LOTS of ammo on a planet is easy, and doesn't slow the planet down.
Third priority would be protecting the shipping, but mostly they are just going to bug out when the scouts spot something.
Given one of my colonies whole purpose is to exploit an asteroid field, and I anticipate having LOTS of small mining colonies, I am going to have to rely on a fast response fleet, which means prepositioning tankers to make redeploys more easy. I have a large number of small commercial jump ships around, where I bulked them out with fuel tanks to act as tankers for survey support. Still kind of mad I lost a full tanker/jump ship to a spoiler because it was trying to rescue a stranded Grav survey ship.