Wet Flies:

First let's talk about married wings, Goose Duck and Swan all marry very nicely together. Also note that most marrying issues are do to using the wrong sides of the feather. Meaning trying to marry a left feather with a right. I have married duck to goose to swan with no issues what so ever. The thing that you need to look for when marrying feathers is make sure the segments from two different feathers came out of the same region. Now why is this an important step. Look at a feather in general. Notice how the quill segments are smaller and thinner in the bottom of the feather and as you go up the feather the segments get a little larger and a little thicker. When making married wings on wet flies keep the quill segments consistent. This will build a stronger wing that ties on the hook easier and prevent crumpling/folding under.
Now once quill segments have been cut out and sized, there is no need to cut the wings. Just tie them on. Before you tie them on measure them on the hook where you would like to tie them in on that makes you feel the wing look right. I have my own method and proportions when tying classic Ray Bergman wet flies. I will not share this in this thread unless some one wants to know.
Now far as buying feathers, I do try to hand pick them. The reason for this because I can see the quality and size of the fly and always look to see on the back side of the feather how large the blood line is. Feathers with very big blood lines are useless and will not give you a lot of usable slips. Keep in mind I tie and fish with size 6 & size 8 wets flies very successfully. So this large blood line gives me a lot less segmnets to use that will fit my size 6 & 8 hooks. Also the quill segments in front of the blood line is usable part that use to form the wing and make the tie in point on. The blood line segment is not to be used for the tie in point. This makes tying the wings in very difficult and also makes the wings roll around the hook.
Now feathers I buy on line I found a few good sources that I use. These shops get to know me and learn what I am looking for. Yes feathers do get crumpled & wrinkled when you buy them or they get shipped. In most cases the feather can be easily fixed by a steam iron on the lowest setting. Anything more damaged then this I send back to the shop for a change out.
Lastly when marrying turkey like in the Ferguson I use goose shoulder instead of Duck or Goose Quills. The Goose shoulder follows the contour of the turkey and allows marrying to occur very nicely.
I know a little about wet flies and I hope this blurb helped out.

Andy B