About Legion of Everblight. Hordes is going to have the final field test pdfs very soon. Maybe a week or so. In that pdf should be quite the same rules that will come out when the Legion book finally hits the shelves (maybe in autumn?), so I'd recommend to wait until that and then come back for them.
For Retribution of Scyrah I could advice you first to pick your favorite caster. All of them are quite different and work in a different fashion. On the same sentence though I could say that all of our infantry and solos (save the Scyir perhaps) are good and will work with any of the casters. So do some research on those caster and come back then and we'll see were to go from there.
EDIT: Also in this very forum we have a good source for Retribution knowledge:
http://www.dakkadakka.com/wiki/en/Retribution_of_Scyrah