marble is a form of limestone so it will raise the hardness of the water, although to what degree I do not know, but best not to have it in there imo. The unidentified plant looks like a type of vallisneria. It's a good plant to have in an aquarium but may struggle in the substrate you have and lighting might be an issue. See how it goes.
Vallisneria Asiatica (Live Aquarium Plant)
Vallisneria sp. Natans (live aquarium plant) is available from Urmston aquatics. 0161 755 0030.
urmstonaquatics.com
my hardiness is way low so hopefully the marble won’t do much damage
thanks again