That would probably work for clay, though a rip-off. I bought like 10lb bags of much higher quality clay for $7.50 each. What I usually do is wait to add the clay til I'm mixing the soil. I get the soil wet, rip the clay into small pieces and mix throughout the soil. others will let a chunk of it dry then sort of grate it in as a powder.
That is a soil that some use... but if you're going the MTS route, GET CHEAP TOP SOIL. Not potting mix. Personally I dont like the idea of mgopc. I've hated miracle grow products in general for years now as their ferts are garbage and can do lasting damage to soil. As you can see in the scott's top soil, it contains peat. We don't want peat. We don't want wetting agents, we don't want added nutrients, anything that's already got "juice" if you will. THE CHEAPEST top soil will do. http://m.homedepot.com/p/40-lb-Topso...180/100355705/
I think the soil varies by region, but again... cheap cheap topsoil is
what you want. Once sifted and gone thru the process, it's a nice sandy/gritty/soily mix, just what your plants want. I'd probably do 2 large bags at least to be safe cuz once you've sifted and mineralized, you will have a lot less soil than what you started w/. You're aiming for about 1/2" - 1" of MTS.
Soil can be messy if not capped properly before fill and also if we're rough when uprooting plants. Otherwise, be gentle and you shouldn't have problems. I also have malaysian trumpet snails in several mts tanks and they do not cause problems.
Personally I prefer sand as a cap since it's easier to plant into, but I've got one tank capped w/ flora-max (similar to ecocomplete's larger gravel bits), and it has done very well. I think the additional benefit of a cap w/ high CEC helps to reduce algae outbreaks around inception. In my experience, it's also less messy when up-rooting for some reason.