Does the DLC offer things that effect sandbox mode if you don't play using minor races? I'm not a particular fan of the Snathi. I prefer to immerse in a serious tone, lol. I saw a reference to an Arnoran starbase. Any chance the Arnorian ranger is back? Any clues?
Not sure I understand the question. But I'll answer what I THINK you are asking.
In this DLC the Snathi Revenge are introduced as a playable MAJOR faction. They can be included or not, if you want, in Sandbox mode. The only way they'd slip into a game where you didn't want them is if you hit the Random button for opponents at galaxy creation.
The other two things added, outside of the Snathi themselves and the scenario Revenge of the Snathi, is the new scavenger ability and the Snathi ship style/parts. Even if you don't like the Snathi themselves (booooo hisssss - burn the heretic
), the Scavenger Ability as well as the ship parts will come in handy. As is access to the Santhi Tech Three, now that I think about it.
If you wanted to, you could create a Custom Race with this Snathi ship style and tech tree and give them some other leader pic and story background. 
As for the Arnorans, no, the Ranger isn't back yet. This is something all new and interesting for this DLC scenario. Paul likened them more to the Peacekeepers (which makes sense if one looks at the innards of the DLC) but beefed up. A LOT.
And while I won't spoil the surprise, there is some nice benefits to blowing some things up in this scenario.
All in all, well worth the five bucks, I think (even if I am getting it for "free"
).