I don't know if a stimulant pack designed for the human body would revive an alien or kill it
What does xcom ufo-lore say about alien physiology? The aliens claim to have seeded earth in order to harvest us "now", which implies a certain physiological similarity, but I guess that is only true for the humanoid ones. Snakemen and chryssalids and celatids and silacoids, on the other hand, I don't think so.
But yes, targeting unconscious aliens with a medkit should be possible, just like shooting them or hitting them with melee weapons should be possible. I don't know about firearms... maybe pistols would work, but standing over an unconscious enemy and lowering your rifle to finish him/it sounds like something that takes a lot of time. Perhaps at double TU cost?