The tank will definately cycle faster if you use the rocks from your established aquarium. But, first thing first, the tank that housed reptiles. Make sure you check the silicon seals. Reptiles have claws (unless it was snakes
and tend to scratch at the tank. Also, silicon dries out after a while. I suggest you put the tank in the bathtub and fill it with water to make sure it's water tight. You may need to scrape out the old silicon and replace it. It's kinda a pain but better than setting the tank up only to have it leak out and flood your house.
I did this a few years ago. I had an old tank that I'd housed a green iguana in. The silicon was junk. I just looked on line for how to replace the silicon seals. Good luck.