I'd probably give eldar slightly above-average WS and BS as a baseline
(Guardians are civilian militiamen, yet are on a par with Imperial
Guard, the Imperium's professional military).
Strength and Toughness are round about human, although I'd be tempted
to drop Toughness down to 45 as an average, to emphasise the physical
fragility of the eldar. Alternatively, Toughness advances are hard to
come by in eldar careers.
Initiative should be pretty high. I'd suggest 70 as an average, or
maybe 60-65. Certainly well above humans.
Sagacity might start out at the human average of 50, but because eldar
culture is more open, intellectually-speaking, than the repressive
Imperium, I'd allow for more career advances in Sagacity.
Willpower should be reasonably high - the eldar are a powerfully
psychic race, and the warlock/farseer career path should amp this up
even further.
Nerve... Guardians are Leadership 8 in the wargame, aren't they? Eldar
are pretty level-headed creatures. Somewhere in the region of 60ish as
a base, perhaps.
As for abilities, I'd suggest Ambidextrous, as it's suggested on
several of the old models (particularly the aspect warriors and the
old plastic-armed guardians) that they don't mind which hand they hold
their weapons in.
Obviously they'd have the ability to use Eldar and shuriken weapons as
standard. Literacy (Advanced) should be the norm, I expect.