I have gone pretty long without war, without an appreciable fleet, using diplomacy boosts and Diplomat citizens.  Not sure how many turns, probably around 100.  But without focusing on diplomacy (or the ideology choice) that is pretty difficult.  Nice job

What I usually end up doing is buying strong ships from the Bazaar if I see that other civs are wanting to "conquer the galaxy" before I manage to build up a respectable fleet. Simply keeping your hit points comparable seems to begin to give them a "message".

Get them high enough and even the Drengin will start to respect you a little bit more.

You'll need tons of Diplo for that, won't you? Some Civs don't seem to consider diplomacy at all...

Any other useful Techs for that pacific goal?

I'm sorry that isn't totally true. I recommend concentrating on diplomacy in the tech tree to pull this off. I do have to research other things too. This is how I conduct diplomacy. If ais are not asking for war, influence techs, or diplomacy techs I give it to them. My income is alright if you concentrate on tourism. I send freighters to the players who are furious, or hostile at me. I pay tribute to the ais who are hostile, or furious at me. I have 60 planets. Under the evil ideology. I take the first bottom. No penalty for having neighbors.

Nonething involving me. This doesn't mean that people aren't asking for things, or I'm not paying tribute.