Just a hunch, but since the diameter of a 50/36 chainring is smaller than your current 53/42, you may find that the current chain will be too long - you may need to take a link or two out of it. Not that you would ride like this, but you'll know when you put the chain on the new small chainring and small cog in the back. If the chain is dragging on the chainstay, the chain is too long. As for the width of the chain, I would think it would be OK on the new chainring.