Yes, mixing things is actually the way nature does it. It is very rare to find a patch of lake bottom that is purely sand or purely gravel. What is more important is that it fit the particular items you want it to work with in the tank. Some sand can pack too tightly for some plants to root well. Some fish may dig and rearrange sand so that the plant substrate is exposed. If the look is not right that may figure into what you want to do.
Sounds like you might like a tank with different areas. No reason you can't do a planted area and have a retaining wall seperate it from the other area. Sand in front right and planted substrate covered with gravel in the rest? Works with my small 20 gallon. At some point the two may get mixed but I don't put my big digging fish in my planted tank! A small bit of sand sifting doesn't matter to me. I used a tile saw to cut the edges off limestone to make the flat edges on the retaining wall stones.
Or to stretch the flourite over a larger area, mixing it with small gravel works. Pretty much like most of the rules in a hobby, if you want to do something a little different there is no way to tell until you try it. If it works, it works and let somebody else worry about the rules.