If you have a large stash, since your space is 6' wide, I would probably place tall bookshelves perpendicular to the railing, back-to-back, with a small aisle between rows (like a library) concentrated on one end of the space or the other, for fabric storage, etc. You can leave them open or cover them with little curtains shirred on a rod. Leave an aisle down one side of the space or the other so you can get to the shelves. This way, messy shelves won't be seen full-face from below, so if you're not absolutely neat and organized, it won't matter. That leaves room at the other end for a quilt wall and whatever sewing cabinet configuration you have, plus some open floor space for a cutting table and comfortable chair, maybe. Does this make sense?
If you have a small stash, I would normally think about cabinets on the wall, with counterspace built below them with cutouts for your machines, or placement of your sewing cabinet below them. That would leave space for a cutting table, ironing board and comfortable chair.
I know you think this space is small, but it is not. 6x15 is 90 square feet. I have about 80 square feet, plus a closet. All this makes you do is put a little more thought into organization and getting rid of stuff you're just hanging on to. Please post pictures when you get it set up -- I might be able to steal some ideas as to how to better organize my space!