I hate to be the bearer of bad news, but if that's what you've got, you've got tertiary or secondary feathers. You can fletch with them, I've certainly fletched with worse, but they won't have the stiffness you really need, nor will they last long.
I wish I had the photo's to show you, next time I buy goose wings, I'll have to do a guide for ZS.
Here's a drawing though:
Primaries are really the only ones suitable for fletching, and usually not all of them. A dead give away is if they are a-symmetrical. The more symmetrical they get, the closer to the body they are, the less work they had to do and so the worse for fletching they are.
They can make great Flu-Flu's though (splicing may be necessary). Don't be discouraged because they aren't A-grade material, seems like you got them for a good price and it's for your own personal use, so if you need to refletch after a couple of shoots, it's not the end of the world.
For goose primary's, untrimmed, they are usually sold as 9" minium, and usually 11"+. You'll probably only get 4" of fletch out of those feathers at best. I'd guess you'll get an unsorted set of bags, but it might be all one wing in each bag. So long as you keep it consistent for each arrow (or if you are really perfectionist, each sheaf of arrows) you're all good.
For splitting them, I think my primitive fletching guide has some pictures (using pigeon... see, I said I'd fletched with worse). You probably won't need to sand or trim the core down much either (I learned a neat way to do it, fast).
You are also right that goose is the traditional flight for medieval arrows, so much so that it's a "Grey Goose Shaft" in some contemporary references.
When I was across the pond, I got a bunch of these (Literally, cheaper by the dozen).
http://www.featherplace.com/feather-typ ... tural.html
I haven't found a UK supplier, nor am I in a position to deal with them, but when I next go on a fletching binge, that's what I'll do. Or order ready made from an archery supplier.