Will, not too helpful linking to a different forum... that does not answer any questions.
have any of you ever used topsoil and peat, topped with sand for substrate? I'm thinking about using it.
There are many, many recipes for substrate mixes, some more, some less successful.
Soil covered by sand is a common mixture for low tech setups. Many have success with it, at least for a while. The soil provides a rich mixture of nutrients for plant roots, including a carbon source which helps in tanks that are not CO2 enriched. The sand covers it all up so the water column doesn't turn green in an instant.
There are certain risks and disadvantages with a setup like that. First off, soil is a term for many different things. Organic, inorganic, different levels of fertilizer all make it difficult to define what soil really is.
Fine sand keeps circulation to a minimum, some plants don't like that. Plus it keeps all the goodies that fish excrete on top of it, not visually appealing, and not usable for our plants. And it doesn't have much nutrient exchange capacity.
After trying a couple of different mixtures, I found one that works well for me. Instead of unpredictable soil, I use peat. Peat is peat. I enrich it with an initial charge of nutrients, using Jobes sticks which provide NPK. I put those underneath a 1.5in layer of peat. The peat is covered with a high CEC (nutrient exchange capacity) fired clay substrate. Examples are Flourite, Soilmaster Select, Schultz Aquasoil, Profile, etc. Actually I found a Kitty Litter (Special Kitty) which works well for me and is extremely inexpensive.