I've decided for sure that I do want to get rid of the sand.
I need to make a decision on the mixtecos. The problem with rehoming them is the chance they will end up better off than with me is basically nil. Once I remove them to empty out the tank, they would need to be rehomed that same day or within a day or so. I don't have a 2nd tank, and I'm not getting one right now. The chances finding of someone competent enough to care for them around here within that small window, even for free, is basically nil.
So that means taking them to the LFS. They would have large gravel while in the store, and if they sold, who would buy them? What are the chances they end up in sand and in good hands? Probably slightly greater than nil. They might just end up getting dumped in with the firemouths (crapiness notwithstanding), which risks hybrids getting into the hobby.
All this means I'm leaning toward just keeping them. Actually, I would cull some of them. There are 4-5 little runts that just aren't growing, and 14 is more than I really wanted anyway. So I may pick the 9-10 best ones and keep them.
Once I get the new substrate down, I'll take another look at livebearers to finish the tank.