I'm not too familiar with GRP Tires, but first you need to be sure you're not buying Nitro Foam's. These typically come in 26mm Front and 30mm Rear, and are listed by Shore Rating. A Higher Shore Rating = Harder Tire.
Indoor Carpet Foam tires are usually 26mm and 28mm and are listed by color for Foam Softness/Hardness. All brands are somewhat different, but a general rule of thumb is, darker color = harder foam. The most common are Pink and Magenta compounds.
Before you drop money on tires, check with the other local racers at your track and find out what brand and compounds they are running. When you go outside of the norm on tires, you could easily end up wasting your money.