What light do you have over the tank? Light level is crucial to plant selection.
IMO you'll probably have better luck with the Fissidens on the wall rather than down as a carpet. It can collect lots of debris, and since you can't have any shrimp to help keep it clean (your Ram would eat them), I'd put it up on the wall where it's a little less likely to collect. Debris = algae, and mosses are especially prone to algae since they're relatively slow-growing.
I'd get some driftwood and attach the Anubias onto that. It would add a bit more depth to the aquascape, too.
Yes, you can switch out the substrate without harming the fish- as long as you're careful to do it in a way that minimizes disturbance to the tank's cycle. Do a thorough gravel vacuuming just before breaking down the tank, and collect as much mulm (debris) as possible. Pour off most of the excess water, and then lay the mulm down underneath the new substrate. The mulm is full of N-bacteria so can really help to get the new substrate cycled and minimize ammonia and nitrite spikes.
Reseve about 50% of the tank's water to put back in the tank, to help reduce the risk of water parameter shifts.
Also, be sure not to clean your filter for a few weeks before and after the substrate swap.