I think I remember reading something in the RAW that said Space Marines were at -20 fellowship when dealing with anyone other than another Space Marine because Astartes feel aloof compared to humans.

Am I making that up or is it actually there?

You may be thinking of a specific chapter trait. I cannot, at the moment, think of any such rule applying to all Space Marines.

Space Marines interacting with other imperial forces depends a lot on the chapter. Many considers baseline humans "in the way" or care little for individual human lives, whilst other chapters (notably Salamanders and Space Wolves) are utter bro's.

It would be strange that a Space Marine would get a -20 to his Fellowship in any case, considering that they are idolized as the God-Emperor's own Angels of Death. You can be as aloof as you want to be or have your head on fire, they'd still think you were the most beautiful creature in creation.

I have hardly opened my Deathwatch books in years, but I'm with Herichimo - it's probably a chapter trait, if anything. Some chapters are assholes.

It would be weird for the Ultramarines to have such good relations with imperials that they get a fellowship reroll as their passive -- and at -20.

(notably Salamanders and Space Wolves)

That depends!

The Salamanders in particular seem to be subject to a lot of "re-interpretation" by fans who propagate their proximity to the native populations as presented in the original GW material - yet at the same time conveniently omit that these very same sources also suggested the Salamanders to forcefully usurp positions of leadership among these tribes, and that these vaunted positions are one of the primary reasons for why locals want to join the Chapter. In Gav Thorpe's story Know Thine Enemy, the Salamanders certainly had a very short fuse when dealing with common people, and though it was a Black Library thing it is of note that the author was partially responsible for designing this Chapter within the main studio.

Likewise, the Space Wolves are pretty much pariahs due to their aberrant ways, and as much as they might be "bros" with Fenrisian locals, I would expect at least potential for trouble when interacting with the common Ecclesiarchy-indoctrinated Imperial Guardsman, both due to the usual "culture clash" as well as conflicting beliefs coupled with the SW's apparent lack of restraint.

It would be strange that a Space Marine would get a -20 to his Fellowship in any case, considering that they are idolized as the God-Emperor's own Angels of Death. You can be as aloof as you want to be or have your head on fire, they'd still think you were the most beautiful creature in creation.

This, too, depends on interpretation. Games Workshop's books say that people are actually more afraid of the Space Marines rather than admiring them. It is an interesting contradiction a la "it's awesome we have them, they are our saviors ... I just don't want to stand too close to one". Quite similar to their religious impact in simultaneously being quasi-heretics and sons of the Emperor at the same time. The very concept of Space Marines is full of such double-edged controversies, right up to the most basic idea of being transhumans fighting for ordinary people to keep their humanity, by giving theirs up.

It may be true that a lot of novels, and perhaps this RPG too, do not care for these things and pursue a somewhat simpler depiction of relations, though. I'd say go with what you feel fits best into your idea of Grimdark vs Noblebright (see the other thread).