First, the easier of the questions. The wheel centers that are on the Camaro are from a toy (or game) called "Hubbs" if I remember correctly. I don't think they make them any more as I got them on clearance for $.25 for two packages that had 4 wheel & tires in them (they did not all match) & removable wheel centers (that I used) Here is a pic showing what was included. I guess my point was to keep your eyes ope as you never know where you may find some wheels to use. I have a friend who found a 1/43 donk that he bought for $2.00 and took it appart to use the wheels on a 06 Mustang.

Now, about Pegasus. A set of aluminum sleeves that turn 19" wheels to 23" come with tires. They fit pretty tight and have a lip to hide the seam. Look closly at the rear wheels on the Challenger and you will see a difference in color (I have not polished the aluminum part yet), or you can count the lines from the center out to the rim on the front, and do the same on the rear to see where the plastic chrome rim stops.
Now about their website, it does seam they have not updated it in a while. I called out there before I placed an order and the guy (I think his name is Mike) is very helpful. You may want to call them with your questions if you don't have a local hobby shop that carries them. This link is a little better. I know there was a better one that showed the sleeves.
http://detailmaster.com/Merchant2/me...egory_Code=PGH
The different size wheels, You need to see the 23" ones and most are based on a 19 version. I know the chrome T's are (the ones you have) and the chrome Hellas (I have a set of each). Your LHS should have a current catalog and be able to help you out. If not, give them a call directly and they are very helpful.
Let me know if you need anything else.