MAgneto vs Plastic Man

Could magneto harm him at all?

Nah, this has been done. There was an epic thread a while back for this....try the search.

http://www.killermovies.com/forums/showthread.php?s=&threadid=355917&perpage=20&highlight=magento+vs+plastic+man&pagenumber=1

There was another too but I can't find it..