Hi Peter,
I think Anthony was thinking that fine sand might not hold up the burrows very well; that there should be some coarse gravel for the structure to hold (at least for the areas that are not under larger pieces of coral chips or live rock).
I do see the randallis using their pleopods to jet sand etc out of their burrows. They also remove pieces very efficiently with their pincers (yes, that's the right word...) No doubt a finer sand would be much more gentle for the gobies...In pictures from the wild, they are often in mud/silt substrate....
Have sent you the email.