As for your sand, I think it's do-able. There's a member who used to have bichirs on PFS and they still looked amazing. Look through this thread you'll see Julian270's nice bichirs on the light sand.
https://www.monsterfishkeepers.com/forums/threads/west-coast-poly.445041/
I think the person's Teug and Dels look best on that light substrate. It's a tan / nautral mix, though, not whilte PFS like the OP is mentioning. I still think that the upper jaws might be OK on white, but the OP should be aware that it might affect how they look.
I have mine on a sort of lighter, natural-looking mix like peace river and mine all look great. (Delhezi, Polli, Moke, Teug) So I don't think UJs need to be on garnet, but do think that white might decrease contrast / wash out some UJs even. But others may disagree with me too.
