The most common reason I know of why people mix a nutrient-rich plant substrate (fluorite, eco-complete, etc.) with a non-plant substrate (plain sand or plain gravel) would be in order to save
$$$. I haven't checked prices lately but IIRC most of the plant substrates cost 3-4 times as much (per volume) as non-plant substrates. So yes, they are expensive.
Of course, the whole point of using plant substrates is that they reduce greatly or even eliminate entirely your need to supplement plant roots with root (fertilizer) tabs. So while you are spending more money up front, you are saving some money over the long haul.
Now, I suppose you might want to mix substrates simply for the visual effect. However, like others have pointed out, mixing substrates of different sizes (sand/small gravel/large gravel) tends to maybe look nice in the short term, but in the long term typically tends to fail pretty badly, as the differing sizes WILL eventually separate out. How quickly that happens depends upon how much distubring of the substrate happens, either from you, from fish, from snails (like MTS) burrowing, etc. If you are going to mix substrates, either for cost-saving reasons or for visual reasons, it is a much better idea to mix two substrates of the same size.
Among the plant substrates, ADA Aquasoil will lower your pH (but take seriously the warnings others have given about it needing to be replaced rather frequently). Many other plant substrates are pretty much pH neutral (fluorite, fluorite sand, eco-complete), and some provide slight alkaline buffering (onyx sand and onyx gravel). Something in the back of my brain is telling me either SeaChem or Carib-Sea also makes a few high-pH buffering plant substrates but...don't quote me on that.
There is really no reason to select a substrate that buffers either acid or alkaline unless there are some very specific creatures you are intending to keep in this tank. Most common aquarium fish are perfectly happy anywhere around neutral, whether the water is slightly acidic or slightly basic. The same is true of most aquarium plants. Attempting to adjust the pH in your tank (through any means) is always a risky business; while using a buffering substrate is far superior than using the pH-adjusting chemicals the pet stores try to sell you, it is still less than ideal.
Note that plant substrates come in lots of sizes and colors. There are sands in (at least) black and grey; and gravels in (at least) red, black, grey, and brown. Almost certainly even more than that, those are just the combos I can think of off the top of my head.
The two main planted substrate companies, and their listings of their substrates:
SeaChem Planted Tank Sands & Gravels (makers of Fluorite, Fluorite Sand, Onyx Gravel, and Onyx Sand)
CaribSea Planted Tank Gravels (makers of Eco-Complete and Fluoramax)
So in summary (since I was so wordy with all this!)...
- it's best to use all one substrate rather than mixing substrates
- if you feel you must mix substrates, it's best to mix same-sized ones (sand+sand) or (gravel+gravel), rather than different-sized ones
- ADA Aquasoil needs to be replaced quite frequently, something not true of any other substrate out there. (Though in its favor, some ppl say it provides better growth than the other plant substrates too. I've never tried it so I can't comment on that.)
- in general, it's safer to use a non-buffering (or very slightly buffering) substrate rather than one that strongly buffers, unless you really know what you're doing and have something very specific in mind you are going to keep in your aquarium
- you can find nice plant substrates in a variety of colors, either in sands or in gravels
Good luck with everything. Let us know what you finally decide!