Seems good so far, needs quite a bit more progress before I put it in my favorite category.
A few months ago I read up on all the implants of a super marine, they're quite op and I really hope he gets to use some of the more interesting ones of them.

The eyes of everypony in the library were on him. He looked around and asked:
"Frightened? Or disappointed?"

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Do you really expect to be attacked here?

Just because he's paranoid doesn't mean there isn't an invisible daemon about to eat his face.

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I can't look at that dude's face without laughing!

Then, she switched to a serious expression and asked almost conspiratorially:
"You are an OC, aren't you?"

Man, I remember that moment in the show where Pinkie stared straight at the camera and shouted "Hey, look at me! I'm a Deadpool ripoff who will constantly fuck with the fourth wall!"

Look, the story is good, but I can't stand it when stories do this. So many good stories with talented writers, and they characterize Pinkie as Deadpool, but small, pink, and hooved. And also less violent.

"Not really," answered the Space Marine, "but we have a proverb: 'The Emperor protects - but caution never hurt either'."

Or a loaded Boltor.

I'd recommend dropping the "we are" "you are" and the like. They sound way too robotic, especially for ponies like Rainbow, Applejack, and Pinkie. Heck, in the second chapter AJ kept saying little sister like a robot, and it just threw me off by how un-texan she sounded.

In short, I'd recommend going over most of these chapters after watching some of the show and reading a few more stories. This story is good, but the lack of proper spacing, the few typos, and the robotic dialogue make it a tiny bit lackluster.

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I agree informalize it a bit for the equestrian characters. We're, it's, wreckon, they're. Ect.

Very nice story btw. For me it feels just a tad rushed, but thats most likly noones fault, I usually just like reading really long stories.

"Of course not!" interrupted Pinkie Pie, "Though it would be great if it was!"

You WANT to be probed and/or invaded? XD

"Hey, you are pretty fast!" Pinkie said, still smiling. Then, she switched to a serious expression and asked almost conspiratorially: "You are an OC, aren't you?"

Wait... did pinkie break the 4th Wall to Antonius? XD

Friends? That was a bit too quick for Antonius.

Ooooh thus will be awesome Xd

Celestia help her.

Celestia cant Help with pinkie....

"I would prefer to sit in this location. It this possible?"

Is*

"I am a Space Marine of the Ultramarines chapter. You should stop doing things like that."

Only an imperial fist could be more deadpan.

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Same