View different layers in different windows?

While I'm certainly no expert, I do have some experience creating masks, using channels and other methods.  The problem I run into is this: I may photograph an animal, cut the subject out from its background, then create a scene around that subject.  In creating a scene, I may pull a variety of trees, bushes, grasses, etc. from images in my catalog.  Often, whether the animal is the subject being extracted or whether I'm extracting some bush or clump of grasses, in any case, there is not a lot of uniform tonal contrast or uniform color difference between subject and background.  So, the usual methods (find the channel(s) with the greatest contrast, apply curves to increase contrast) these methods are of limited use.  I'm trying to bring up my skill level to where I can use tools like calculations, blend modes, etc. to make more effective masks and to minimize the amount of handwork I need to complete or to clean-up the selection. That's why I'm reading books on channels (and Scott Valentine's book on blend modes).

Also, I seem to be giving the impression that I use selections instead of masks.  Rather, I use masks to make my selections.  That process (making the mask) most often involves several layers.  This is an issue because my "base file" (the file which receives all of these cutouts, blended into a scene) can become very large (sometimes approaching 40GB).  One method I might use to control this is to develop a system of "support files" (suggested by Chuck Ubele a la Bert Monroy).  Leave the several layers that comprise the mask in the support file.  Make the selection (the cutout) in the support file, then duplicate that single layer into the base file. If I need to modify that selection, I can do so in the support file and re-export to the base file.  I'm only now developing this method because file size has become an issue.  

What I don't understand is why you don't use a mask instead of a selection copied to a new layer?

With the selection method, as you describe it, you're burning all the bridges behind you. It has to be perfect the first time because you can't go back.

Using a mask instead, you can refine it to your heart's content as you go along. If you discover it's not perfect, you just paint in the mask until it is. That's your live preview.

You can also turn on the mask rubylith overlay as you work:

Sorry for the ambiguity.  By "selection" layer I simply meant the layer containing the result of the selection.  I might select a subject and do a ctrl+J to put that cutout on a new layer, i.e., a "selection layer."  Yes, that is somewhat sketchy.