I have sand tanks and gravel tanks and I have cories, and as long as the gravel is small and rounded it is fine for them. Some gravels have sharp edges and that is bad for their barbels. I love sand but it has it's issues so if you are relatively new to the hobby you can skip it for now!
Think of water changes as taking out the trash. The fish use the food and the bacteria in the tank use the waste that the fish produce, and the bacteria in the end produce nitrate, which nobody in the tank can use, so it is up to you to "take out the trash" or the whole place will deteriorate, much like it would in your own abode if nobody took out the trash, or waited forever to do it. For most tanks, it is never a bad idea to do a water change. I do them about every 3 days for the most part.