Hi Concept kidd.
I can give you a bit of advice.
First of all let me tell you that the cars won't show up until you hit the track, in the car selection you will see only the car you are replasing. If you already knew this lets go on...
I seem to share some of your problems.
I will send you a file for MK's program that is ment to tell you wich cars will freeze your game. I was told that MK's program will show cars that are only available in GTC 2002 as hidden cars for GTC 2001. The file that I will send you will tell you wich cars are not in the game even do the program displays them. Don't try this cars because they will freeze your game. I have only been able to unlock only two cars. Don't know why but the others that I have try, that are supossedly in the game keep freezing the game. I have unloked only the Skyline R34 Pace Car and the NSX Hashiken version. I put the NSX in slot one and it worked fine then tryed to add another hidden car and it froze. I have tryed to put just one hidden car in slot one and have not had any luck with the exeption of those two mentioned. I do have been able two use the first and second slot for those cars in the same save. So, are situation is not entirely the same.
Wich cars have you been able to unlock?
If you manage to unlock the hidden cars please send me a save, I will keep trying, but my PS2 has been hijacked by my girlfriend to play Dance Dance Revolution at this time.
When you get the file that I'm going to email you replace it with the one that is in MK's folder, then try it out.
Hope I could be of any help.
And by the way Welcome.