poly/cotton blends wear like iron and do not shrink or fade...and minkie is polyester...so, if you can use minkee why cant you use the fabrics you found? there are no quilt police; back in the olden days quilts were made with scraps of everything available. i know , there wasn't any poly back then but...crazy quilters use cotton,silks, velvets, leather/suede...ect. the poly would be great for a kid that is going to drag it around for years ...
all that being said...blends and polyester does not soften and fluff with washing like 100% cotton does, so there is the downfall to using it.
but again...no shrinkage, no fading, wears like iron...