If you are using GW pieces and bits:
1. I would use Chaos Marauders or Catachans as the base model for an unarmored marine. If youre up to it I would also use green stuff to make the torso slightly different as marine physiology is not the same as a human (no rib cage as its solid, exaggerated musculature, sockets for the black carapace, etc).
2. For the Eldar look in the fantasy lines (wood or dark elf), Mordheim Amazons, or possibly use the mandrakes. There are also a few torsos in the wych box. You could also use on of the assasins if you want the body suit look.
If youre not restricting yourself to GW parts then there are plenty of options for the Eldar (warmachine/hordes and reapers line would be a good start).