I like how you have kept that far left rock in the same place, that's a perfect positioning. I'm thinking you should really try some left-skewed or right-skewed scapes and see what you get.
I see of course what you mean about using an odd number of rocks to avoid a symmetrical scape; there is irony in the fact, however, that you've managed to make a very symmetrical scape using an odd number of rocks. Quantity's not related to symmetry -- arrangement is!
Whatever you do, do whatever works best for you. I'm not sure if I'm bothering you, but I feel like I am
"Simplicity is the ultimate sophistication." -- Steve Jobs
I think those plants will need more time to form into a dense carpet. I would go for HC, because they grow much faster. Just my 2 cents. I have no experience with those plants anyway.
I've never had much luck with HC and I don't really plan on fertilizing at all. Maybe a little co2 here and there. I also don't really want my hands in this tank either which isn't really possible with the occasional trims that HC requires. Moss and Belem would be a lot less maintenance.
Nice rock formation! I like the idea of a fissidens carpet but I agree that it would take forever to carpet. Why not just follow the idea of a belem carpet with sections of fissidens mixed in? It'd all grow together pretty nicely.