Envoys increase your relations with enemy factions, though you'll need a ceasefire first.  They also have a few other researchable abilities which mostly help the ally, the TEC envoy can make you fairly hefty sums though.

If you research the first three levels of relations bonus you should get offers for ceasefires soon enough, either from destroying enemies during the course of normal play, or from completing missions that ask for resources or cash.  If you're playing an unlocked single player game with a few AI good relations should be your priority. 

If you have both expansions you have everything.

The pirates need a rework.  They can be fought off by good players, but if you were playing at the Hard AI level the pirates would be far more of a threat than the AI, which is why players complain.  If you want to try a pirate mod here's a link http://datefilehost.com/download-448aad49.html  Put it into your mods folder (the right folder might be hidden) and enable in-game.  

(If I insult your intelligence on the game here, I appologize.  The way I'm reading this you are new?)

Every Planet has TEC militia guarding it, which must be removed.  The better the planet, the better the militia that must be removed.  Terran and Desert worlds will always need a full 90 supply of 1 cap ship and as many Light Frigates(LF) as you can build, or just 90 Fleet supply points worth of LF if you don't want a Cap.  (Not recommended, but some people don't build them, or just build the first free one)

As to "how do you use them?"  I assume you mean envoys and Pirates?

Envoys:  Research them, build one, and send it somewhere with an Allied/Enemy planet.  (Preferably one you have at least a cease fire with)  Research its other abilities eventually to boost the ammount of "Like" that you get from them.   (As TEC, always research the ability that gives you credits, it's like having a small colony you can tax, that is crazy cheap.  But only one envoy should be at a planet at a time, unless you are needing to overlap something like the Vasari's Planet Invulnerability thing on an ally...)

Pirates:  Research the option in the diplomacy tree that allows you to offer missions to pirates, open the Criminal underground tab up top next to the research tab, and click on pirates.  There will be an offer mission button.  From there, it's self explanatory(I don't remember what exactly it looks like, but it's easy)

You can also research something to offer missions to other factions.  The higher the reward you post, the more likely they'll do it.

As to the one tech strategy, that's a bad idea.  YOu should have 2 of each lab up as fast as possible.  (3 Military if playing Advent...Abd I believe TEC, but I may be wrong, to get Long Range Frigates(LRF) out quickly.  Early game, you will need these ships.  Next in line build Flak Frigates AND Light Carriers to counter the enemy LRF that will come shortly thereafter.  Beyond that, the game evolves in too many directions to predict.  As TEC, the Probe ability is very useful, and a first tier Civ research.  Also, Terran population upgrades are an emmence help early on, but only if you think you have time.  They'll boost the population cap on your homeworld and any other Terran planet, but on the Homeworld, you already get a credit bonus.  Added onto the fact thatthe homeworld is hopefully the last planet the AI will be able to get to, you'll have a decent chunk of your economy with which to try to get back into te game.  (Though you may well lose anyway by that point, but I'm rambling)

Starbases are also something to get out fairly quick, but definitely after LRF, Flak, and Light Carriers.  (Idealy, you'll have 7+ planets before a SB goes up, and have bought the first, and MAYBE second fleet supply upgrades)

If you see an asteroid in your early scouting, grab it.  It returns credits from its esearch faster than any other planet.  Essentially, their purpose is pure economy Support, though they can have other purposes in a pinch.

If I didn't answer your question, please be specific.

-Twiligh Storm

You can upgrade each planet's logistics slots.  Your homeworld maxes out at 24(Which is to say six structures, since they all take 4...)

Select the planet and click on the button to the right that looks like a planet with a grid on it to find the logistics slot upgrade.

In Sins, focussing on one Tech tree is a difficult way to play.  The devs did a good job scattering the good techs between all 4 trees.

The Uber Unit in Sins IMO is the Starbase.  Other than that, a Fleet of Light Frigates, Long Range Frigates, Flak Frigates, and Light Carrier Cruisers is a nearly unstoppable force if you and the enemy are on equal ground fleet wise.

-Twilight Storm

Terran and Desert will always have large fleets, but as long as you are alright with losing all the escorts for your cap ships, and remember to replace them at the HW as they die(Preferably with LRF) you will almost never lose these battles, even with the starting 100 supply points worth of ships.  Focus fire LRF and Kodiaks,  and use your abilities on these ships to take them out fastest and that grav well is yours.

You need to expand very quickly, at least until you have 7+ planets, then start pumping up upgrades.  Before then you only want to research civilian infrastructure to earn an income.

-Twilight Storm

I didn't mean to imply Starbases were the be-all-end-all unit, simply that they are the toughest unit.  I agree that LRF and Carrier Caps have a massive presence, it's just that to me, Uber implies big nasty unit, and Starbase says that to me.

-Twilight Storm