You have to use 16" wheels with ST170 brakes. That's why it gets that funky orange alloy for the spacesaver spare. Even then some will not have sufficient clearance.

As you know, the standard knuckles have to be changed usually as they, for some reason, do not follow normal Ford lug design, but the ST170 does. Whilst it has the standard calliper lug dimensions, albeit with M12 bolts, the offsets are different.

This can be illustrated, for example, by the need to use 10mm (?) spacers when using ST170 discs and callipers (or Mondeo Mk3 callipers) on an Escort knuckle. Going the other way onto a ST170 knuckle with another Ford calliper might be difficult.