I am not familiar with that batting, but your plan of having quilting 1/4" on either side of the the square seam lines should not be enough to make the quilt stiff in my opinion. I have quilted baby quilts like that and they came out flexible and soft. The quilts that I have that are stiff, have much more intensive quilting designs than that, like small 1/2" pebbles, etc. I think you will be fine with your original plan.