Hi Joe,
Welcome here!
Thank you for appreciating this humble forum.
To form a nice carpet with rootless plants like Riccia and Mosses, they need to be weighted down and evenly spread over. You may use inert material like stainless steel mesh like below picture.
For your focal point plants requirements and small aquarium size, go for small low light plants like Anubias barteri var. nana ‘Petite’ which you can tie them on wood.
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