OK, the important part 1st. You CANNOT just get a set of alloys from a 450 and fit them to your 451.
The offset of the rear alloys would make them hang out of the rear arches by about 2+ inches.
You have to get 4 front alloys. The 450 front alloys have an offset fairly close to the 451.
Unlike the old type fortwo (450), the new fortwo (451) has similar offsets front and back.
16" Monoblock VI
16" Monoblock A
The most narrow standard wheel on the rear of the 451 is 5.5" wide. So you don't want to go any more narrow.
The alloys listed above have front widths of at least 5.5"
Standard tyre sizes for suitable 450 front alloys are OK all round for the 451.
175/50/16 (5.5" wide alloys)
195/40/16 (6.5" wide alloys)
Thanks to Neil for the photo of his car on Spikeline alloys.