Welcome to SSD, Rebecca!
To answer your question...for challenge layouts, you can use ANY products and post them to our gallery. For non-challenge layouts, though, we ask that they use something from SSD designers.
(PS, we WISH Kay was a designer here...she was but only for a couple weeks, we're TRYING to convince her to come back, hehehe...she IS one of our "Sugar Babes" though, so one of the CT members here at SSD)