Daisy has given you some excellent advice. My perspective here...
I don't repot most of my cacti except when they need more space. I do provide regular weak nutrients when I water, so I'm not relying on years-old cactus soil to provide that for my plants. If you do unpot them and replace the soil, do it very gently and be sure not to water for at least a week afterwards. Every time you do this there is a risk of rot if damaged roots get submerged too soon.
The only thing other than soil components that I usually leave at the bottom of a pot is a small square of window screen just big enough to cover the hole. The rocks at the bottom do not typically work to your advantage. With some pots that have lots of holes I might put a single sheet of paper towel cut to size, and the paper towel will disintegrate over time.
The issue of salt buildup in the soil can be avoided by flushing every time you water. If you make sure that some water exits the container every time you water (say 1/4 of the total) and does not get slurped back up, you're providing an exit for salt so it does not build up.
I prefer to water in more than one pass (separated by a few minutes), rather than leaving pots in saucers full of water. But the goal is the same: to water to completion. You could wait until they have slurped up what's in the dish and then water one more time, emptying out the final pass through at the end. More on the process here.
The thread "Sunday afternoon experiment: watering in multiple passes"
in Gardening Ideas forum