Flourite will only release the nutrients very slowly. In fact it will serve you for quite long. If you are going for carpet plants like glosso and HC I suggest it, else you can manage without it.

Regarding algae, it will still appear even if you have normal substrate UNLESS you find the right balanace. Moreover the substrate will help you reduce the ferts in the water column although with that amount of light your plants will need ferts don't worry

Glosso will surely do well in a tank with ferts, high light and pressurised CO2. You can also try HC. It will enhance the aquascape with its small leaves. Riccia will become a pain in such high light from my experience.

Regarding substrate I strongly suggest a plant substrate such as Seachem Flourite or EcoComplete. Plants will love it. A 10g tank will not need more than one bag.

Very nice speciment! Well done

This scape has lots of potential! gl mate!

I have to agree with rams. It looks like an E. Stellata in emersed form. It will surely change but do not worry if it starts dropping leaves suddenly. The important thing is that it starts developing new leaves.