The Eldar
The Eldar are a tricky subject matter to address but non the less were around at the time of the Great Crusade and are an important race in the game.
Although this is mostly assumption hopefully the following will provide the Eldar player with a plausible structure from which to craft their armies.
The Fall
The Fall is the most crucial historical event to the Pre-Heresy Eldar and also plays an important role in the creation of the Imperium.
Before the Fall Terra had been isolated from the colonies by violent warp storms that made Warp travel all but impossible.
As the Eldar decended into complete madness by their most base desires some of the Eldar fled the Crone worlds in great ships known as Craft Worlds.
At the peak of the madness the emotions of the Eldar became material in the warp giving birth to Slaneesh. A psychic shockwave tore across the universe as the God was born killing all the eldar in it's path along with many psykers from other species.
Only the Craft Worlds that had reached the furthest away from their homeworlds had been spared.
The terrible birth of the God scattered the Warp storms and mankind once again took to the stars under the guidence of teh Emperor.
The Eldar in Warhammer 30,000
The main difference between Wh30k and Wh40k armies is determined by the Fall.
At the time of Wh40k the Eldar have established the Aspect Shrines and intregrated the Aspects into their armies.
However, at the time of Wh30k the Eldar were still recovering from the destruction of most of their race being that it was only a matter of two hundred years or so after the Fall.
This presents us with the concept that not all the Aspects were in existance. Asurrmen had begun to create the Dire Avenger shrines and had possibly taught Jain Zar by this time and then these shrines would most likely be restricted to the larger Craftworlds.
Dark Reapers are unlikely to have been around untill three hundred years after the Heresy as Maugan Ra spent this time apon the craftworld of Altansar before it was sucked into the Eye of Terror.
Therefore it's best to break the Eldar into the following catagories:
Craftworld Suggestions
The most varied armies of the Eldar are going to be the largest Craftworlds to survive the Fall. It is most likely that Asurmen visted these first to spread the Path of the Warrior and so they are more likely to contain more variation of Aspects.
Autarch 0-1
Whilst the Path of Command is still a long way off it is fair to assume that the warriors of the Eldar would be lead by a forerunner to the Autarch. Most possibly this would be the most experianced Exarch (perhaps known as the Young King?), a Farseer or a determined civilian leader (which may be the case on smaller Craftworlds).
Wargear Restrictions
If you are using your Autarch as the most experianced Exarch then you should restrict your wargear to one single aspect.
So for example if you wanted a Dire Avenger Autarch then you would be restriched to weapons that define that Aspect such as the Avenger Shuriken Catapult or Power Weapon.
Remember that Dark Reapers, Swooping Hawk, Striking Scorpion, Fire Dragons and Shining Spear Aspect Shrines were moet likely just begining to be created so it's best to leave their wargear options out.
For Guardian Autarchs you would be best allowing only the power weapon, Avenger Catapult or Jetbike Upgrades.
Aspects
This is a tough one. For major Craftworlds I suggest that Striking Scorpions, Fire Dragons, Swooping Hawks, Dark Reapers and Shining Spears be restricted to 0-1.
For Minor Craftworlds I'd use only Dire Avengers and Howling Banshees as the only Aspect Warriors have Howling Banshees as a 0-1 choice.
Avatar of Khains
As the Aspects weren't fully established it's unlikely that the Avatar would have been in existance this close to the Fall.