Thanks for signing up. You are right that the GT86 and the Scirocco are not GT3 cars but the OP states that 'All cars participants use must be exact to the FIA GT3 specifications. Any other cars participants wish to use must be race cars that have been used in the real 2013 edition of at least one of the races.' Sorry if you didn't understand as it probably isn't that clear but if a car someone wants to use isn't a GT3 car but has raced in one of the real life 2013 editions of these races they may but it will need to be made equal with the GT3 cars therefore making it 'exact to the FIA GT3 specifications'.
The Ford GT race car isn't a GT3 car, it is just PD's own Ford GT race car so it isn't eligible. It is similar to the Z06 LM and ZR1 LM corvettes in GT6.