One: I don't currently use a UV sterilizer, some people swear by them, my 2c.
Two: The longer the water sits in the UV, the better. I would recommend 200gph-ish, and a union on each side of the sterilizer.
Three: Plumbing it to the skimmer output would depend on the model of skimmer. The reason I have heard for that is that UV breaks up organic molecules that you want the skimmer to remove, preventing the skimmer from working. Now, I don't completely ascribe to that, because most skimmers are only going to remove a fraction of the pollutants in the water, takes many passes to get everything out. Thus, if you ask me, UVs are not worth it. BUT, if one wanted to do it, yes, probably attaching it to the skimmer outlet would be beneficial. How you would do that would depend on your model of skimmer.