you could mix them, but they will separate into layers eventually. larger grains always make it to the top due to smaller ones falling between the cracks. in this case, the larger grains are lighter too.
if you want a good plant substrate with plenty of ferts, use natural (organic) planting materials and cap that with something heavy and smooth. aqua soil grows plants like crazy, too.
other options are to layer root tabs and such right into the substrate of choice. a good cec and fe content don't exactly count as "soil" in my book. even under water...
there are plenty of things that can be used as caps. you pretty much have your choice of natural colors if you know what you're looking for. lapis lustre might be available to you beings you're on the west coast. then you've got fluorite sand, estes sand, aquarium gravel, eco, tms, pool filter sand, play sand, etc., ...
you could also use kitty litter (just make sure the ingredients list is clay... period... just clay...), oil-dry, coco-fibers, worm castings, leaf litter, etc. for or to amend your soil to your liking.
if you are going to use something with actual fertilizer, you might want to consider mineralizing your soil before your inhabitants are introduced to it. this can be done in a separate container if you wish.
if i had to choose between the 3, (eco, fluorite and fluorite sand) i'd say loaches would appreciate the sand better most likely, but eco shouldn't be an issue either.
that's my story and i'm sticking to it.
