i don't have advice about the foot, but i'm wondering two things. 1 - did you have the layers basted sufficiently, either with safety pins or thread? typically i would hand stitch a gridline about 4" apart to anchor things. and 2. you mentioned on the sample you stitched the edges first - there isn't any foot that's going to prevent wadding up if the edges are stitched first. even with a walking foot & basting your layers may move some. with multiple layers you always want to first baste it thoroughly and then start in the middle and work out. the bigger your project is, the more important it is to start in the middle.
for the diamond straight stitch pattern you've got, i'd literally start in the middle of the sandwich and go to one edge, then return to the middle and stitch the straight line to the opposite side. i'd probably do a couple of lines like that to get everything anchored thoroughly in the middle, then i'd switch to going the entire way across in one line, checking as i went to make sure nothing's gathering up. i'd do all of one half before returning to the middle and working out from there to quilt the opposite side.
i have a really good walking foot on my 503a and i always baste first and start in the middle.
does that make sense?