People have different preferences for thickeners. For fruit pies, I prefer tapioca (either tapioca starch or instant tapioca ground to a powder). Conversion would be 2 tsp tapioca to 1 Tbsp flour - so 8 tsp tapioca instead of your 1/4 cup of flour. Tapioca will also stand up to acidic fruit better than cornstarch. It is also somewhat more forgiving than flour if you add more than the necessary amount (to a point of course). Flour can be pasty if you overdo it. Another alternative thickener would be ClearJell. You can also sugar your peach slices, let them sit for a while and then drain off the excess juice to reduce the moisture content.