Just make sure that your inner ring clears your chainstays if you go the route of reducing your bottom bracket spindle length. Some frames will not allow this kind of reduction.

A triple FD will work on a double. The reverse is not necessarily true, however. It depends on how small your granny gear is and that the back of the FD cage crossmember stays below the chain while the chain is on your lower gear combinations (like the granny in front and the third from innermost or so on the cassette)