I do not use gravel for my succulents..either pumice or insoluble crushed granite..or sometimes I also add kanuma or akadama..if I have them on hand.
Our growing environments differ..so it all depends too on ones rain frequencies, seasonal light and heat levels.
My area gets cold down to 30F overnight and we get some winter rains but no snow. So it becomes imperative to use cacti mix with added pumice and a thin layer of gritty material to protect further the base of the plant and the leaves touching the wet media.
During our long hot and dry season when temps easily soar to 100F..having gritty media and gritty top dressing allows me to water them generously and let them dry out faster, so their roots have good airflow easily. Semps are quite shallow rooted too, so it will not want to be staying too wet at the rootzone.
