gday mate
this is a good start too! you have had some great advice, especially from craig. the odd number of treees will make it look more balanced, even numbers dont generally work aswell. thickness of the trees should vary somewhat like craig has said too. the thing with using buxus is that when they are confined they will be quite slow to thicken. shoot out quick but thicken slow.. so if you feel the need to change it up thats up to you but slow growing trees has its merits aswell, craig will agree with this.
if you are going for a gum look with the buxus then consider how some gums grow. they can be straight and tall until they get to the branches then they will have angles kicking around alot. I dont know how you will get severe angles in buxus as they can be a little bit of a bugger to bend as they are brittle. what you could do is get a few more, get them in a grow box or even better the ground, and individually grow the trees for the setting, craig actually put me onto that too, I have a beuatiful landscape tray but am not prepared to use it yet as my chinese elms I plan on going in are only in a seedling stage. elm take forever to thicken in a pot. similar to buxus. so patinece is key.
as for putting ferns and other rooted plants in as accents be careful as these can actually suck nutrients away from the trees you want to get the nutrients.
good luck
