Okay

Ive heard a few people going on about using various combinations of artefacts and then the spcial quest tiems to make 16x mega colossals etc..

How does this work? I now have my terraformers and am curious as to how I should go about making this planet, is a gas worth it and load it with the injectors etc or?

Checked the wiki couldnt find much

Anyone got any solid info on how to get the most out of this?

Also: can these planets be stolen?

You need more then just terraformers to get a planet to 16x Mega colossal.

Essentially thier are soft limits that you can get a planet to using terraforming devices and quasai space expanders. These limits are 15x resources and Very Massive size. Thier are some other artifacts such as a mass surface metlers amongst others that increase a planets size and production and which will push past the soft limits.

Gas planets afaik can't go past these limits, you can get quest items for Icy, Terran, Desert and one other (?) so to make the most of these limited quest items what has been said in other threads is to use the terraformers and quasai space expanders to create a 15x mega very massive Icy, then you can use a mass surface melter to turn it into a Mega Colossal 16x.

And yes if someone finds it and is strong enough to get past its defences it can and will be stolen, a planet that good would need to be very well guarded.

They can be stolen, and they are only certain types.

You can make Icy planets into Mega Colossal Oceanic planets, and you can make Desert, Barren, and Terra planets into Colossal planets. Basically, just use Terraforming Devices until they are 15x Mega Rich in at least one resource, and then use Quasi-Spacial Expanders to make them Very Massive, and then use their artifact, Mass Surface Melters, Gamma Crystalizers, Planetary Sentiox, and Gaia Seeds, respectively, in order to push them over the Very Massive 15x Mega Rich limit.

EDIT: And you'd probably need to buy the Quasi-Spacial Expanders to get enough.

Im referring to the quest item you get from handing the 10 terraformers in

Whats the best way to get my best bang for buck out of it

That's completely different. You can't make that Colossal or 16x Mega Rich, you can only push it up to the limits of the Expander and Terraformer. You can give it additional passive attack by using it on a Gas or Volcanic planet that you've used Gas Igniter Rods or Eruption Channelers on, but you needn't bother, because that planet actually can't be taken. Technically it could be, but if you have at least 20 people in your Legion it should be invisible, and I don't think anybody could invade it with the 10,000 passive attack and defense. You could try using it on a planet that has a structure with a doubled effect, just to get a little more out of it. For example, put a Convoy Heat Seeker on a Volcanic planet so you get 2 Mining instead of one, and then use the Hyperterraformer on it and it will retain the 2 Mining of the Convoy Heat Seeker.

So i can use 7 eruption channelers on a planet, load it with that passive cloak crap too - and then use the hyperterraformer? which would give me a 10k planet + 350 attack etc...?

you could chose any planet to change and its cloak alone will be so high that no one will ever find it anyway so there would be no reason to worry about any extra attack or defense for it.. have not done the mission myself but i know people who have and none of them have ever had anyone scan or hack their game made planets from them

I whipped up a quick wiki-page.

A recent update changed it so you can use 20 of them on a planet, but as I said, it's not really necessary anymore because it gets 10,000 passive attack/defense/cloak.