How is Sword of the Stars: Gold Edition DLC different from the "regular" one?

In the spirit of this question and in view of the current Humble Weekly Bundle:

• Does the Gold edition change the game forever? Is it rather an option at launch or in the settings screen? I know Steam doesn't normally allow you to 'hide' the fact you have DLC purchased for a game.
• How does the difficulty change? Is the game with the DLC objectively harder, easier or just "randomer" than the "vanilla" game is? Is it meant to be a "new-game-plus" content pack for the players who've seen everything from the game?

Although it's been a while since you asked this question, here is my two cents as a person who played its vanilla and fully upgraded edition.

Gold edition (and mindgames dlc) do change the game forever. You cannot turn them off and on at will.

The difficulty... well, if we can put it poetically the vanilla version is a beast who cackles gleefully while beating your dead body with your own arm. It utterly, utterly hates you and shows you this in every single step of the game. You are playing an engineer, you hope to open some crates and reprogram some computers, right? Well too bad! In 7 of every 10 attempt you will fail pathetically and the remaining three will give you cheese. If you are lucky. I mean, my vanilla characters were often killed by their own stomachs instead of being perforated by some other alien or clawed to heck by an animal. Hunger, ammunition, poverty... these are your enemies in the vanilla version in my experience. I mean this game will give you a badass assault rifle but conveniently forgets to give bullets.

The gold edition is... I mean after you survive (or rather get used to) vanilla version it feels like you are headed for an epic grade clusterfrag. Why? Well, my good sir, I never got used to get useful stuff from my crates and containers let alone opening them with a crowbar in 90% ratings! And seriously! I even got a pistol which shoots shotgunshells in level 3! Either I am going to die in a horrible fashion which will be told as a horror story to our next generations, or random number god is tamed with the addition of new items and classes (which I believe so).

Sorry to make it this long but seriously, if you are able to do that do play its vanilla version. See if I was wrong about my assessment about its diamond grade hardness and hate in the scale of AM in "I have no mouth and I must scream" by Harlan Ellison then load up the DLCs and see the difference. I fervently hope that my vanilla runs were just specially unique to myself because it was tear jerkingly, insanely frustrating in the extreme.

Mindgames and Gold both add in new characters and content to the game. These are things that, so far, I haven't been able to toggle on or off through the game.

Mindgames introduces you to Psychic abilities. These are things ANYONE has access too, but some classes are better at it then others. This, in turn, adds a new stat and six new skills to the characters to track how much psychic energy you have (much like HP) and your skill in the six classes of psychic powers. You are not required to have psychic powers, but it's always an option for your character.

With the new psychic abilities, you now have two new classes to choose from. The Solforce psychic starts with a little combat ability and a lot of psychic powers, but is pretty low in other places. The Tarka Ranger can't wear heavy armor, but she gets three movements every turn and fast healing. She also starts with a large amount of combat ability.

With the Gold edition, you get three new classes to start with. There's a Hiver warrior, which starts with a flame thrower and is a beast at combat. A Morrigi Striker, which has a bit of combat and psychic power, but also has great stats for manipulating robots. You can also use the Liir Seeker, which is mostly a glass cannon in terms of damage and usage. There is also the unlockable characters, Sargent Gunny, which appears if you're having a REAL hard time with the game (die a number of times). He starts like a marine with a lot more skills and weapons.

Added into both DLC, there are a lot of new items, monsters, weapons, devices to use, ect. And there is a new mechanic which 'helps' people to start at lower levels. Every five floors, you will encounter a blue room with an Exp storage machine and an item storage machine. If you reach any of these floors (5, 10, 15, ect.) and have extra items and Exp, you can store that into these machines. What this means is that if you want to start a character off on floor 5 rather then floor 1, you can. And, when the game begins, they are able to take any Exp and items from these machines to start their playthrough. Notice, however, that these things are gone after you take them with a character. And you cannot take from the blue rooms as you reach them. You can't start a floor 1 character and have them reach floor 5, then take items from that storage area. It only works for the character that starts on that floor.

All in all, I don't think I would say that the DLC makes the game into a new game + difficulty. Though, because of the added monsters and psychic abilities, it does make it different and possibly harder for some people. I find it 'differently' challenging when I started playing Gold edition.

In response to the first question, yes. Mind Games, Gold Edition, and now The Pilgrim, all change the game permanently, there is no option to turn on or off particular pieces of DLC at launch.

As for the second question, I don't think it can be described as objectively harder. It adds some new enemies but none are significantly more difficult than existing monsters, just more variety. It adds some new equipment, but with the RNG the way it is, you could go through a whole playthrough without seeing them. It adds some new area types, but as long as you avoid them like the plague (which is an option), it's no harder to deal with. It adds 4 new characters. Two of them are significantly more difficult to play as than the existing characters, while one of the others is significantly more powerful that any other character in the game. If anything, it makes the game easier, and in a good way. The new items it adds expands the usefulness of certain skills, which may not have seen much use previously. The Spear and Knife skills, for instance, got new, stronger weapons. There are more food items available, and more recipes to create food, which reduces the risk of starvation. There are more items to increase your inventory space, which reduces the frequent problems faced with that. I think it does add enough new content to be worth picking up and act as more than just a "new-game-plus". However, if you have played the no DLC version, and enjoyed the extreme difficulty of such, then I would recommend not picking up the DLC, to maintain the existing difficulty. I also believe a big draw is for those who have enjoyed the other Sword of the Star games, who may be interested in playing as their favourite races (three of the new classes are from different races you could play as in the other games).