Interesting that you think the Scott's Premium Topsoil is all wood strands and chips. I just bought a bag of that stuff and I think it's going to be a good soil to use: no fertilizers, additional peat moss, screened to remove chips / stones / sticks. I've also used the Miracle Grow Potting soil too w/ good results.
I've made several soil based tanks and usually just add the soil dry to the bottom and then layer the gravel on top before adding any water and planting the plants. If you decide to compact the soil layer by wetting it and making it mud, be sure not to add *too* much water. Personally, I've had more problems w/ soil sublayers after they get wet; gravel sinks very well in wet mud.
The main issue w/ soil sublayers, as far as I have found out, is indeed trying to keep them covered w/ gravel. I have had issues w/ muddy waters usually when I don't add enough gravel on top and the soil sublayer peeks through. The solution? Add more gravel to the thin areas and be sure to drain off all the muddy water. Then refill w/ ~2'' of water and check that it is clear and not muddy. If it is clear, finish filling up the tank completely. If it is still muddy, drain off the muddy water (again) and then refill w/ another ~2'' of water and check again. Repeat as necessary (I once had to do this 3 times total for a particularly muddy tank, but usually only have to drain off the muddy water once). Good luck
