You may modify any pattern you obtain for your use. What you may not do is use patterns designed by others to create a new pattern to distribute to others without the designers permission even if you give the designers credit. I will give you an example.
This cutting
Elements of this cutting came from two other designers and parts came from me. I can not share this pattern with anyone because I do not have permission from the other designers to do so. I can cut this pattern all day long and show off the cutting all I want and do as I will with the cutting. I just can not distribute the pattern in any way shape or form.
So you can continue doing what you are doing with your layered peice, you just can not share, post, sell, give away the pattern you come up with, unless you obtain permission from the pattern makers who designed the elements you are using.
I hope that clears things up a little, and that I did not make you brain go crazy.