I'd like to purchase "Masks of Nyarlathotep," but I can't seem to find a definite answer on seventh edition compatibility. Does anyone know for certain?
The changes between editions of CoC are generally pretty minor. While 7th edition is one of the biggest changes, converting material designed for older editions is not difficult.
All you need to know to do so is covered by Appendix II: Converting to 7th Edition Rules, a 6 page section near the back of the CoC 7 book (that has lots of art, examples, and some big tables to use up most of that space).