Yes, a 16" rim will fit over the ST170 front calliper, after all thats the diameter of the 'orange' spare wheel.

The offset is really more critical to handling in keeping the contact patch reasonably centred on the steering axis, its really the profile of the back of the wheel which as with any calliper is the issue.

The Focus Zetec 16"x6J, ET52.5 is OK, and you can get away with Cougar 16"x6.5J, ET45.

Martin