Hi,
I don't have a guru license at all (maybe a learners permit, that's about it). I was wondering why you were wanting to mix the Eco Complete with the topsoil at all. It just seems more trouble than it's worth, unless it's for cost, design or aesthetic purposes. Eco Complete is enough to grow you plants brilliantly as is. I had a look at the substrate link you provided and while there is a lot of interesting data, there seems to be one overlooked point; chiefly, based on your intended configuration of 3-5 WPG and CO2 injection there is no way soil alone is going to satisfy the growth demands of your plants properly. You'll have to dose the water column, presumably using EI. If that's the case, the plants will then feed predominantly from the water column because they are aquatic, and therefore you really needn't worry too much about finding the perfect substrate.

If you were attempting a Non-CO2, low light tank, then yes, water column dosing would play a much less prominent role and the soil property would be a preeminent concern, but that's not the configuration you have selected. I think that it certainly won't hurt the plants to use topsoil but that things may get rather messy when you try to uproot plants to change things around or disturb the substrate in any way. Life is so complicated I would hesitate to make it more so as there are more than a few easier soils such as that which you mentioned, the AS and the Eco Complete, both excellent.

As I said, if you enjoy using the topsoil or if there is a cost concern, or if you are just wanting to experiment then that's another issue altogether, but based on what I see from your intended configuration it just seems unnecessary.

Cheers,