Second can I put these in a tumbler and see if that will clean them up? Anything negative that will happen if I do that?
I've done this with several rusty dies—even some very rusty ones purchased on ebay. Granted, this is with stainless steel pins in a wet tumbler. Worked very well, and no harm done. Nothing in a tumbler is going to be harder than the tool steel they make dies from. I'd just hesitate to put more than one die in at a time, or to have decapping rods floating around in there with the dies. I've always tumbled every part separately.
