I have mud and sand with cheato. The bed is 3-4 inches deep. When the cheato grew crazy, I added hermits to thin it out. At 5 months in, I have large coepods all over. My tank has 3-5 inches of sand mixed with live sand, plus rock. No live rock in sump.
I don't want to use mud because I'll have to always be taking it out and replacing it... I currently have a powerhead on the bottom, some egg crate about 6 inches from the bottom and a few pieces of rock... Wanted to put some sand, some rock and cheato... Some nasarius snails to keep the sand moving. Kind of like a mini tank
Unless you go deep sand bed, theres no reason to use any sand in the sump refugium at all, as it gets in the way of cleaning, gets stirred up and makes things a little messier moving stuff around. Your rubble is much easier to manage. If anything, you can fill it up with more rubble and just leave a few inches of free space above it for macro algae.