I do like the idea of recovering objects and choosing whether to keep or sell. Especially with alien weapons and grenades. Even alien power systems and such.
I would say the simplest solution would be: If the player can use it in some form (even if not the most ideal way)...give it to them, don't auto sell. If it has no in game function for the player...auto-sell it.
I'd suggest the same with alien corpses and captives to be honest.
maybe have different alien leaders and commanders have different info, so random lottery of what your interrogation turns up? Or other alien classes to get other info? (Alien engineers, medics, navigators/pilots etc.)