Huh, I have never heard anything about them not fitting with the Brembo brakes. I installed the street starter kit on my HPP Ecoboost (brembos) with no issue whatsoever. AFAIK they just don't fit convertibles
The S550 Eco PP was all the same size wheels as is the S650 Eco PP... the S650 has a different design/pattern on the Eco PP vs the GT PP so it's easy to tell what they came off of.
PP wheels from an Ecoboost will be all 9"... and GT PP fronts are 9"... all of them are still PP wheels. The only way they would "not" be PP wheels is if they were some other design/pattern. Don't see that as getting taken unless they specified the actual sizes.
Sounds like a pretty good deal. Never hurts to try for less, some of my best deals have been when I was happy to walk away (and on cars sitting on the lot for months).
I see your dilemma now... that is quite a bit bigger... if you had 30 series on the 20" wheel, you would be good. Everything is fine now I take it, no rubbing anywhere? If all is good, I think it would just be a matter of wait and see... it could just be fine.
Wheel size doesn't matter... the overall diameter of the tire is what matters... as long as the outside diameter of the 20" tire is within 3% of the diameter of the 19" tire you should be fine with either one.