[wkndracer]

Sand would work its way through your soil or mix with it (IMO). In too deep a layer it can also interfere with the gas exchange. What too deep would be I couldn’t say. Ms. Diana is the wizard on this stuff. When I started planning my tank pool sand was my intended capping material but after reading posts I switched.
The bacteria and filtering question I’d also like to post an option on. Reading the book and website information the primary purpose is to provide circulation in the tank. What I use is a power head connected to a DIY spray bar equipped with a Marineland reverse flow sponge kit. This moves the water well and removes any large partials while leaving everything for the plants. It’s very easily serviced and is working quit well so far. Other than the power head I used an air driven sponge to help the tank over the hump as the soil cycled converting to a submerged state.
Hope you jump in and start a tank! Best of luck!