Are 90 spec small bumpers compatible with 90 spec big bumper arches?
Not really, when you remove the bumpers or arch trims from a 90 spec car you'll see that the arch trims are designed to mate with the big bumpers to keep it all rigidly (sp) in place.
The big bumper arch trims therefore have a lot of extra material in these areas and this will all be left exposed if you were to fit small bumpers. They will fit but it will look a bit odd.