A trade back and Mathews would be fine with me. Clowney and Mathews are the only players I see guaranteed to start game one.
Picking Mathews at 2 would be over drafting him. I just can't see Fisher using this rare opportunity on Mathews if he can't get a trade back. The Rams are too good at taking rawer lineman and training them up.
Taking Mathews over Clowney will never happen. Taking Mathews at 2 will also never happen in my mind. If you want him, and I do, you have to stake it on both Clowney going first AND a trade back IMHO.