is it safe to add potting soil to my cricket cage and also will they lay eggs in it?

Answer 1

Yes and Yes. Potting soil would work perfectly as a breeding ground, just make sure it stays moist and warm. They won't lay eggs if the soil is dry.

Answer 2

It is better to add it in a tupperware container that they can climb into. If the whole bottom of the tank is soil, it will stink and be very difficult to clean.