a couple of questions
1-is the 500 credits a one time deal of each peice--500 c's per pants--500 c's per shirt--etc
2--or is it 500 c's unlimited forever?--for all peices of apparel?
Unfortunately, apparel does not stop at 500, or Level 9. From there, you go forward to the Level 10.
You start all over again from 1 to 1,000 and once at 1,000 you are offered the chance to buy the Level 10 apparel for 100,000 coins.
As for the 2nd part of your question, once you reach "Level 10" on apparel (not 500 c's which is level 9) you can go no higher for that apparel. That is why you will see, if you play Head to Head in a coin game, that the opponent's apparel stops at 10.
Hope this helps.