You can easily fit a few more bromeliads in there as long as none of them are underneath another plant. They can be attached using hotglue if they have woody stolon bases or you can attach them to the background using some wire around their bases--they'll grow roots and become more stable over time. You could probably get some moss to grow successfully, but be aware that moss requires more light than most people expect so they'll need to be in both a high light location and a location that does not get too dry. You can try purchasing mosses that do not require any dormancy from plant dealers online, you could try using aquarium mosses, and you can also use moss that comes on plants you get from a greenhouse. I've used mosses from greenhouse plants and aquarium mosses successfully.
You want to be careful with imitators--they can be very aggressive to individuals of the same *** and keeping more than one of each *** in a smaller enclosure will almost always result in one frog perishing as a result of stress from bullying.