Alternatively in my game china seems to be the strongest power, their greater pop seems to have allowed them to avoid a cash crunch like I have experienced and I'm sure the other powers are affected by, they seem to have the greatest tonnage in shipping too.
Actually, china may cause the death of everyone, they're upto their third gate building ship, I don't actually see them working anymore though, after gating sol I've lost track of them.

Population sure have quite the huge impact, indeed.

Though quite easy to organise in one of Steves games,  just put a Chinese colony there...

There is already a chinese colony on my AAR. xD

Except I just blurred the lines more by making steve post some mechanics here XD

What about methane or ammonia breathers, I'll assume that oxygen can be swapped out for those, as long as either of the other toxic gasses aren't present?

There´s any way to edit the NPR stats after they are created? or do i need to create them as player races and then transform them into NPR, is this even possible?

Oh, and how do i add construction brigades on SM Mode?

I'm pretty sure you can't change them after you've created them, though there's 'space  master options' on the intelligence window I can't access due to my resolution which may it may not be related.
You create NPRs in one of two ways, either under the spacemaster option on the main menu there is 'add extra NPR' which adds a new system somewhere with an NPR set up on a suitable planet, you can't edit this but it'll be created in the same way as any NPRs generated during system exploration, so further along the game it'll be larger and higher tech.
OR, under the system information window(f9) you can setup a new empire on any habitable world, that means any world which meets the requirements steve just listed, use the 'create empire button' then you get a window allowing modification of their name, theme, economy, tech points, etc, you can also change their max atmosphere pressure, gravity deviation, oxygen pressure deviation and temperature deviation.
Most importantly there's an option to create them as an NPR. 
If you want to add a new empire to an unsuitable planet you'll first need to terraform that planet, create a colony there using a pre existing empire and use spacemaster options to modify the planet. Oh and you should use teh 'homeworld minerals' button to make sure they don't run out and die early.
Finally adding construction brigades first requires adding the tech for building them, then after turning on spacemaster mode, go to ground unit training, select the unit type you want then press the 'add unit' button.

A new NPR home world may be created during system generation if a planet falls within the following ranges:

Oxygen atm from 0.07 to 0.40
Surface Temperature from -40C to 60C
Gravity from 0.4G to 2.5G
No Dangerous Gases

The environmental tolerances of the NPR will then be set with their home world as ideal and a random range either side.

Steve means destroying the planets ability to sustain life :s.
Anyhow, there is no limit to the population size a perfect planet can sustain , though growth does slow down significantly when it gets high.
I have a related question, but it's somewhat of a spoiler.
What are the requirements for a planet to be habitable to an NPR? At the moment I'm just searching every low cost planet; but I'm sure there's a better way.

"Steve means destroying the planets ability to sustain life :s."

That´s too extreme.

I have a question also, if i set the NPR generation to a 100%, does that means every system i create (via SM) that have a suitable planet will have a NPR? or is it limited to generation through JP exploration?

I find glassing the planet usually works

What do you mean?