I have bought clothing at Goodwill and felted it....and pieces do come out differently. Some shrink to a very small size and others not so much shrinkage. Shrink it first, is what I did. I made a tote bag with these. I cut my pieces, butted them together....don't overlap....and on the tote, I used embroidery thread and did a blanket/buttonhole stitch around each patch of wool. On the tote, I placed all my patches on a backing of muslin, and did the embroidery. This might give you an idea of how you could adjust for making your blanket/quilt.
Sewlady