For the Bonsai tree pass by the local plant nursery and take a look at a "Boxwood".
Some house plants are actually poison. Will give it a shot but Defenbachia is one such plant. Some have no reaction to it and others need to wear gloves to handle it.
Well the Boxwood is in that category. Doesn't in the least bother me but neither does that other plant. But if left to dry out completely, the sap is where any problems are.
I just think the structure of this plant will work for you. As for the size...the smaller ones will have fewer limbs of course.
With that CO2 and fairly high light, especially in the center, you should be able to do some red. Alternathera reineckii comes to mind as a favorite. Been wanting to try a
Cryptocoryne becketti which for some reason most of the pictures of it are mostly green, but I saw one that was purple. Bet that depends on the old either/or age or nutrients or both. BTW I haven't had one in ages but/w most rooted plants you can
only break off new plantlets, not cut them in half and actually you need to be gentle about how you plant a crypt. Amazon swords get huge. A couple of other swords stay smaller. A plant that will be red grown under optimum conditions and that can go across the back, at least on that end where the bulbs are towards the back is Rotala
Macrandra. Not an easy one though.
I do think different shades of green change the look as much as red so I'm not obsesed on red. Just think it gives one more different look to a tank.
You might want to consider that having two of those "walls" in there and placing them opposing each other seems sort of "set-up" and replacing one with a couple of pieces of driftwood might look more natural.