Well first off pacts are extremely powerful, especially the high level ones (for example, TEC supply pact gives you and your ally 1000 tax free supply!). So whether you're playing against people or AI, getting some pacts going will have a big impact on the game. You should try and put an envoy cruiser at all of of your allies planets. They have abilities that give great bonuses like increased extraction rates or tradeport income. Also TEC envoy cruisers have an ability that generates extra income for you. In 5v5 mp games, the diplomacy techs are mostly used by the person(s) in the econ spot, but can certainly be game changing. 3rd tier isnt really that high considering you need two civics labs just to be able to capture ice/volcanics, and at least 2 to get tradeports and culture.

Pretty much just leave em and all their abilities (or if you just want a specific ability turn the others off) on autocast and they'll do their thing. Though you should probably set them up to alternate casts if you want to use more than 1 at a planet because some of the abilities don't stack, but most the time that isn't really necessary.