I mean it is feasibly possible but we would run into issues during the Great Crusade since the Legions didn't take kindly to human worlds ruled by xenos.
Plus the only advantage of having humans in my eyes is manpower since we can pretty much do anything they can do better.
Yeah the only things we get out of having Human auxiliaries is - at best - a bootleg imperial guard (a waste of resources and lives since we're aiming for quality instead of numbers) and at worst, we just get a whole lot of chaff and end up getting the Imperium angry when they inevitably roll around (which they will since we're right next to Isstvan).
Plus, what kind of folks would we be if we hide behind auxiliaries? That's what our armour is for.
So Human diplomacy? Yeah, as long as we don't do something stupid like telling them where our craftworld is or formally creating an alliance like the Interex, Diaspora or any other Human/Xeno alliance of the GC, then we'll be fine (business partners and customers maybe, if we can find something we both want) but getting Humans to fight for us? Making official alliances and forming federations? Bad idea.
Also, not related but I don't want to see this turned into "Humans: The Quest, except from the POV of the Eldar", it's boring and ultimately will just turn into us babysitting them while they do everything, we get plenty of that from Games Workshop as is.
Uhm have you seen our warhost roster? It's basically a high school lacrosse team compared to a full SM legion.
We'll be wearing power armour as soon as we can make it (next turn) so us being a lacrosse team won't be for long.
Directly quoting the flavour text from Char-Gen about our odds by the way.
[ ] Major (-4 points)
You are counted among the largest and thus most influential Craftworlds to survive the Fall, standing equal to the likes of Ulthwe, Beil-Tan, or Alaitoc. Might, you shall have, allies, you shall have–and enemies, too, for your bucking of norms. Even at your weakest, nothing lesser than the full might of an Astartes Legion—Primarch included—or equivalent could hope to lay you low, and lesser foes would break themselves in trying.
[ ] Large (-1 point)
While not as vast and influential as the Major Craftworlds, you are still a cut above the average size. This lends you weight, strength and influence you would not otherwise have, yet conversely frees you from many of the burdens of politics; too large to be casually dismissed by the Major Craftworlds, yet not large enough to be considered a pressing political threat to any of them. Your defenses at their weakest could turn aside an Astartes Legion, so long as their Primarch didn't get involved, with very good chance of success; even in the case of the full might falling upon you, you would have at least even odds of victory.
So as we are right now (in terms of purely our craftworld's defensive ability alone) we have a pretty good/guaranteed chance of surviving a legion trying to kill us (less of a chance with their primarch if I'm reading it right)
This doesn't really contradict anything I've said. Yes, there were instances of Xenos ruling over humans during the great crusade but there are other races that were exterminated for no reason. From what I recall of the great crusade the Imperium's policy whenever they came in contact with Xenos was to exterminate them with no reason needing to be given.
Funnily enough, one of the things I remember about a minor xenos species (I can't remember if it was 30k or 40k) was that the Imperium discovered a xenos species who's body's basically made a way better rejuvenat (life-extension medication) that was basically several times better than any Imperial version, and they ended up harvesting them to extinction, iirc they weren't even all that threatening / they were a peaceful xenos species.
(This might not be canon, but I do remember seeing it)
Edit: Found it! And turns out it's also during the Great Crusade too.
Adarnian - Warhammer 40k - Lexicanum
So yeah, we probably shouldn't get any ideas of empire-building when the GC starts rolling around the northern edge, if anything I suggest we immediately go about trying to either hide or push deeper into the Halo Stars until it's time for the Heresy.