yep plant pots in terracotta are fine, give them a good clean and you are off. plastic may not be so good, high grade pipes and the like are fine. some if not most cheap plastic items, like pots and dishes, can react with the water in a tank. simply because it is so hard to find out which is safe, i would stick to clay pots and pipes.
Around the world clay plant pots are the most used "caves", by Crayfish keepers. cheap safe and readily available.