When all is said and done, it comes down to one's personal tastes. So the upshot will be that you'll need to weigh any given advice against what you personally find appealing.

For example, I have seen many dish gardens/bonsai with the little ceramic fisherman and the like. Personally I find such décor tacky and detracting from the overall presentation. Yet I know other folks who love them. Simply a case of different strokes for different folks.

So some random thougthts:
If the fern turns out to enjoy its place in that set up, it will likely "explode" into growth at some point. Not only will it fully fill out the area it is in, it will try to take over the entire pot. Ferns tend to be "thugs" in favorable conditions. This is not to say that you should rip it out on-the-double, but rather a "heads up" for what you may expect in the future (completely redoing the planting at that future time so you can prune the fern -- and even the boug back in line). Btw, depending on the actual space available in the pot & if you have a suitably smaller container, you could consider potting the fern in the smaller container, then plant the container in the bonsai pot. This can restrict the fern's "runaway" tendencies.

Personally, I would suggest some sort of top dressing to hide the perlite. This would be done with any or all of the following:
*decorative gravel/stones
*a thin top dressing of moss
*planting Selaginella

NOTE: Selag will often try to grow a bit above the soil -- sort of propped up on the adventitious roots. Placing something to press the new growths down against the soil until they root can overcome this.

I would put two tiny ceramic figures on either side of the big rock. Like a frog or a little man. Or maybe a cool looking rock on either side?

Thanks for the input, Paul,

I am planning on growing moss as a top dressing (or at least trying to), maybe with a pile of rocks to form a back wall sort of structure...
I am looking into sources for moss - buy, collect etc... And weighing the pros and con's. I have a huge patch behind the house, but am not sure about bugs, and bringing it in from -16C... I figure it can wait until after I visit the nursery this weekend - maybe they have better bugs ;-p

I am undecided on the fern, same as the bougie, if either likes the planting too much, I'll end up with shrubbery. On the other hand, if I manage to 'control' them, they could be awesome. Come to think of it, the mini trailing violets have shrub tendencies too.... I better leave some pruning scissors close by... ;-)

The rules say plants have to be planted inside the container, so the pot-in-pot idea would probably not fly,unless I can find a way to make it part of the scenery.... Hmmm....

Anyhow, I'll keep you posted. :-)

Well if the fern's pot was planted in the main pot then it would still be "planted inside the container." Wouldn't be any different than if you created an underground "rock retaining wall" to restrain the fern. I find plastic containers work best as you can trim them down so the top of the restraining pot is at or slightly below soil level and thus not an eye sore.

IME, mosses can be quite slow to take off. I have volunteer mosses in my terrs and it has taken years (literally) for them to cover most of the terrs' floors. While you can easily find moss at reptile shows, said mosses are typically cultivated for the poison dart frog trade. As such, those mosses are grown very wet and very humid and do not adapt easily to drier conditions. Floral shops or craft stores may stock dried green sheet moss. This could be soaked and then laid/arranged over the 'nekked' ground areas. Even though said moss would be dead, there would still be the benefits of:
*hiding the perilite
*creating the illusion of lawn
*if kept moist, it might provide a suitable substrate for airborne moss spores to germinate
*reducing erosion when you water
*slowing down soil drying out (could possibly be a two-edged sword there but with all the plants you have drawing water from that same dish of soil, I doubt it)

Btw, it is possible that the mosses you have outside might not do well indoors. Being temperate species, they might require the seasonal changes just as our temperate trees do.

Yeah, who knew there was that much science to growing moss!
I found those recipes, where you're supposed to blend the moss with yogurt, beer or buttermilk, but no one ever actually says they work. In fact, plenty of people say it doesn't. It sounds like a nice "mad scientists experiment" but I am not looking to grow gray fuzz on that dish garden...
I'm thinking other options, maybe av seeds, or sprouts, or rocks :-)
I'll go shopping this weekend to see what other plants are out there....

Sprouted AV seedlings sound like a unique, natural, horticultural addition that no one else would have. Also, if the trailers start to fill the dish , you won't need much else. I am in favor of natural looking settings. A nicely shaped rock in attractive colors might be nice to add to create the look of the natural habitat , the rocks AVs grow on in forests.

If you wanted to try a completely separate look, and have a second entry, you may want to consider an Asian bonsai style as there are many props available. Such as attractive dishes in fired clay pottery with gray- blue-tan matte glazes and fired clay figurines in gray- blue- tan colors , quite subdued, that make a nice miniature arrangement. If you don 't have an Asian Village type of store near you, probably there are products available on line.