>>96579010
I mean it's not really that complicated.
You have all the information necessary for the factions and how they're -supposed- to operate and what the reasonable expectations are. Those provide templates for encounters, and notions for what you could reasonably deal with.
You also have the fact that 40k, as big as it is, is full of gaps and undocumented holes in its universe that are specifically meant to be filled by players. Up until recent years 40k was very much a homebrew game. Being somewhat cheaper than it was now, players got really creative coming up with their own space marine chapters and their own eldar craftworlds, corsair bands, ork freebooters, etc. Errata and rules gradually shifted to favor buying specific armies because it helped move models by giving it a competitive advantage, but that's relatively recent (like 4th edition onwards).
That's actually where the spirit of 40k RP exists. Rogue Trader is sort of the best about this. Like you aren't expected to be spending time in the canon planets. Because of 40k's narrative structure people often forget that there are like regular, even decent planets out there, because whenever they show up in the books or games they're about to go from 0 to fucked in a day or two.
So nah, the deviations are extremely common for 40k RP if you're playing it normally, and not with like, a massive autist as your GM, which is a big ask nowadays.
