Agree, I also use PFS sand, and plants do well in it.
And although I know nothing about Flagtails, I like to keep plants and cichlids, which have much the same vegetation destroying rep.
My suggestion would be to set up the tank, at least a month or 2 before adding fish, plant it heavily, so by the time fish are added, the plants are very well established.(or just add some tiny fish that are innocuous to plants)
I did this with my last 2 cichlid tanks. Planting heavily, and only using a couple dozen mosquito fish for about 2 months.
This method sufficiently cycled the tank, while allowing the plants an initially unmolested and secure time to get a foothold.
The mosquito fish didn't last long once the cichlids, and others were added, but were a great distraction from the plants
