you can make a scrappy quilt any way you want to- if you need a foundation- muslin is fine- or any thin fabric
if you want to just sew your blocks together you certainly can- or you can use sashings/ and borders- really depends on what type of blocks/design you are planning- there are hundreds/thousands of scrappy quilt patterns- perhaps browse some galleries and find a pattern that appeals to you...if you want to just start sewing scraps together in a 'crazy quilt' style you probably would not need sashings/ or borders-it just depends---and it is all personal choices