Could the Imperium accept [i]any[/i] aliens?

[OmegaChief]

There are unquestionably examples of half eldar half humans even in the modern cannon. There is not, however, a full chapter of half eldar space marines.

I'm going to have to ask you to provide evidence of that there claim, as this does not fit with what I recall from cannon at all.

[Cykeisme]

I guess in the earlier days (circa Rogue Trader till 2nd Edition 40k), 40k was really more "fantasy in space" rather than taking itself as sci-fi of any sort.. hence half-elves and stuff like that.

I would imagine that GW retconned all that stuff out because it's more sci-fi now, and the Eldar are a completely alien race, despite any similarities in appearance.

Even taking into account Old One tampering, humans still trace their ancestry all the way back to the origins of vertebrates (and life in general) in Earth, whereas Eldar were developed separately; they completely alien life forms.
Not only would attempts at interbreeding not produce viable offspring, it would likely produce nothing, even assuming Eldar had similar genitalia, or in fact even had anything approaching a rough analogy to human sperm and ova cells.