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.
I think any moss would look great with a sencond small plant peeking out here and there, like 'belem', 'parva', or Narrow Leaf Micro Sword.
A carpet of fissidens? I just fear that it'll take forever to fill in. Especially una setup without any CO2 or Ferts. I find that it'll be worth it though. How about downoi instead of Belem?
Any other suggestions with flora? Having trouble thinking of more.