I think you are off to a great start!!!!
Id add a nice log in there somewhere tho. and maybe adjust your terrestrial layer to have more niches such as some dips and valleys and such.
Personally Im going to say that Pilea would get too big for the foreground.
How about trying Pilea glauca. Its got a great creeping habit. And wonderful colors blue/silver leaves with red stem... awesome.
Heres a shot of mine in my D. tinctorius 'Bakhuis' viv. it tends to grow upward first then a few months passed and it seemed to settle down and grow a little more compact (Not as compact as youd see in the greenhouse but still fairly low. 3 inches max?)
Its growing intertwined with Begonia 'Red Planet' in the lower right hand corner.
Another great filler plant that would stay lower and cover the ground is Selaginella
Great plants. Although they can be slightly tricky to get started... once they establish the fill in quickly
And another great plant which Id consider a slow filler/accent plant are Begonias.
Choose these wisely as some dont like dart frog viv conditions and some get really big!
I think some good proven types would be.
Begonia 'Red Planet'
Begonia bowerae var nigramarga tiny little Begonia (Stay away from the standard Begonia bowerae as this gets big)
Begonia 'Buttercup' tiny little thing.
There are many others.. typically when choosing I go for the miniature hybrids as they are smaller and more tolerant of wider range of conditions as opposed to most species which are adapted for more specialized conditions.
I wouldnt add more than 1 of each of these Genus to the terrestrial layer.
As far as your background. Id need to know the lighting specifics.
But great background plants that would cover and look good with the broms would include
Higher and more intense lighting
Dischidia, Hoya, Creeping fig
Choose these wisely as they could become out of balance with the broms if you choose a species with too big leaves.
Lower to moderate lighting
Peperomia, Marcgravia
On the note of the Peperomias. Choose smaller species that creep trail or ramble as opposed to the broad leaved upright growing species.
I might add specifically Peperomia prostrata looks much better with some air movement and slightly lower humidity.
Todd