The disc machining wasn't due to the rim size (I've got 17's) but due to the position of the mounting points on the steering knuckle (closer to axle than on ST205)
You will have to do things a little differently as my front suspension is based on the ST20x model Celicas, which is slightly different to the ST18x models.
Like DynoMatt said, you'll probably need spacers behind the disc to get them to sit futher out to line up with the calipers, whereas I had to move my calipers out to line up with the discs.
I don't know if the ST20x front struts & steering knuckles (fwd models) fit the ST18x.
Might be better off changing these over (you'll also have the original stuff to refit if you want to sell the car. A problem I'm having at the moment)