I have worked on a few projects w/ deconstructed sweaters (mostly chicken clothes for my naked chickens) and have combined fabric quilts w/ crocheted blocks.
When you deconstruct the sweaters do not cut them at the seams take the time to remove the stitching.
One big problem will be the irregular shape. You should get 4 good size pieces to work with, front, back, 2 sleeves.
If I were going to try and turn them into a quilt from there I would measure all my pieces and decide on a size square to work with ( as large as they would accommodate).
I would cut fabric squares to size and perimeter stitch probably a double row w/ small stitch length, leaving a turning spot, then cut extra sweater away (use the finished edge of the sweater as the "turning edge").
You will now have same size squares w/ sweaters on one side and fabric on the other all raw edges will be turned in. I would join those squares w/ a whipped stitch or X by hand w/ yarn, or a wide zig zag by machine.