I would not add enough peat moss to do that. If you want to use peat moss at all, just a dusting under the other substrates, not even enough to hide the glass.
Sure, mix PFS with the soil, but remember that one of the reasons for using soil is for the CEC, sand has none. Try small amounts, like 25% PFS + 75% soil, see if that gives you the soil structure you want. Is your current mix (with STS) still not loose enough?
I have ended up with a mix of Soil Master Select (similar to Safe T Sorb) and PFS. That was interesting: The sand was just heavy enough to settle through some of the mix, but it actually stayed too well mixed to separate when I took down that tank.
I have also done a mix of coral sand and Turface. This was great for only ONE reason: The Turface removes carbonates from the water. Blended with coral sand, it did not. It looked bad. It did stay well mixed, though.
If you are looking for a sand-like additive, and you need something to add a little bit of minerals to the system, you could try some coral sand. If there is any doubt, though, do not blend it with the soil.
75 gallon... footprint of 4' x 2'? At 2" deep this is only 10 gallons of substrate. Did the soil shrink when you treated it?