Hi all I'm just finishing my 2nd playthrough and have some questions regarding privateer fleets: 

Is is possible to blockade the developing outpost of a rival faction? Or even destroying/raiding the outpost? 

It is possible to blockade a rival faction outpost to slow it's growth, camp enemy food ships and even destroy it. Titanium slugs can help u to destroy it faster.

But do I need to declare war in order to do that? I mean if I use privateers?

With Privateers you should be able to do it without declaring war. You could even do it to your allies or people you're at Peace with.

(it's Frogsquadron's specialty when we do multiplayer tests)

cool! I'll try it tonight. thanks for the replies