Can evacuation be used as a tactic and do evacuated soldiers get experience?

A mission will succeed if you evacuate when all objectives are completed. However, Guerilla Ops always require you to defeat all hostiles as part of the objective. You will only succeed on missions, where you have to evacuate anyway and in those the evac zone is usually fixed. So it is feasible to snatch a VIP and run to the evac zone and succeed without killing all enemies.

In guerrilla missions like stealing data, you will fail, because you did not complete defeat all hostiles.

Evacuated soldiers will get experience for the kills and mission experience as normal. The evac zone can be called on any mission without a fixed evac spot. Whether you can actually shoot an enemy, drop the evac zone on your ranger and get her out I don't know.

Yeah, you should be able to do that.

You can call for an evac zone on any mission that doesn't have a preset location for it.

You might need to drop the zone before moving the soldier. I know for certain that you can move into an evac zone, shoot, and still evac even though your turn would normally be over at that point.

Soldiers don't get experience at the end of a mission. They level up based on their number of kills, and also kill assists. Kill assists happen when they're on a mission and somebody else kills an enemy. I'm not sure whether they would get credit for kill assists on kills that occurred after they evac'd or not. There's a mod out there that lets you see their kill assists in the barracks, so that would be one way to check. See this answer for more information on experience.

If the other soldiers finish the mission, then the mission will still succeed.

I wouldn't use this tactic except in the most extreme of circumstances. You'll lose the soldier for the rest of the mission. Perhaps if the soldier is already so hurt that you're worried they will die, and you can't heal them, then this would be a way to get a last shot in before they leave.

Note that the Alien Hunters DLC (May 2016) brings a few changes with tactical impact. In this case, it is no longer possible to drop the Evac grid over a soldier.

It does still work to place the evac grid where your soldier is about to run to, let them run in there, take their rapid fire or whatever, and then evacuate.

Another nifty tactic using evacuation applies in the situation if you've got one medikit soldier on a mission, but that soldier is the one who gets critically wounded and is a couple of turns away from bleeding out. You can choose one of your remaining soldiers to pick up the one who's unconscious and bleeding out, then evacuate just that one. Your remaining soldiers will have to do the rest of the mission with a reduced squad, but most missions only have 7-12 enemies anyway.