Minimal Defense

Reasonable questions, and as someone that's happy with peaceful coexistance (not the same as pacifism) I think I can help out.

The first thing I do after I've dropped a constructor on every resource I can beat the AI to (yes, I'll even dive into their territory to grab one if they didn't grab it first) is to continue building constructors, sending them out to the established starbases, and using them to add defensive modules. At most points in the game, a starbase can hold off against anything but a concerted attack simply by having all of the defensive modules available at that level of tech.

When you first create a starbase, it has a defense of 3 and no attack. This means that if it gets attacked, it will have an effective attack of 1 and defense of 3, and 100HP. Three Corvettes will take it out.

However, even without any tech, a starbase can be expanded with two defensive modules, taking it from a typical defense of 3 up to 11, which greatly increases it's survivability. After you get the Shields technology, another three modules become available, adding another 7 to the defense, at which point it will usually take multiple dreadnoughts to take out a starbase. The exception being, of course, someone that has a nasty set of bonuses on their dreadnoughts, so you can't assume you're safe.

Two more modules come available when you reach Avatar technology, and two more at Overlord technology. By the time you reach Overlord Technology, your bonus is an outrageous +62.

In case you're not familiar with how to expand starbases, you just have to move a constructor onto the starbase, and you'll get a prompt asking you if you want to expand the starbase. Say yes, pick the "Defensive" modules, and then pick a good defensive module.

Sensor Drones + Eye of the Universe = 18 Sensor Range

The only practical approach to minimizing defenses, which includes starbase defenses since they aren't free, is to scout extensively. If you can spot the enemy 30 turns before they arrive, you can easily respond to their incursion with a small centralized fleet. I often manage to survive with 2 Corvettes and a Battle Axe until Dreadnoughts start rolling off my production lines, even on the highest difficulties. The AIs tend to get a little jumpy when you only have minimum defense of course, but it is more of a temporary annoyance than a serious threat as long as you capitilize on the resources that are spared from military production and maintenance.

combine this with the "starbase catapult" and you don't need a huge force.

There's another good point. If you trade alot, including tech trading, you'll maintain good enough relations that you'll get left alone. In my current game (huge/challenging) I have yet to build a single military ship. The Drengins threatened me once, so I gave the Antarians (directly between the Drengins and me) all my combat tech, and while it wasn't enough to kill off the Drengins, it was enough to remind them that they weren't the only major military power in the galaxy. I'm still at the bottom of the military might chart, but that's fine by me, even at 10% military/10% social spending, my manufacturing is twice anyone elses, and if they do threaten me again, I start cranking out overlords. Which, combined with my two fully upgraded military resources, would be very bad news. Even my constructors have 256 HP. I may send out some Excaliburs to rack up a military score before I get the tech win.

The safest way to remain a pacifist is to make sure you have the strongest military. The AI won't declare war on you if you have sufficient military might, and military might from what I've read is calculated by adding the att + def values of all your ships. These values include any bonuses you get.

So the trick would be to find the ship that provides the most att and def per maintainance cost, and to mass produce them so that your military rating will be sufficiently high to deter any military action against your empire. Races won't ask you for tribute either so it's a nice bonus.