First off let me say that this is a fantastic idea, and I'm incredibly glad that someone with the know-how is going after it! I'd played around with this idea but have always dismissed it before I could get to excited over it, as I don't have any experience with the art of Bonzai growing (YET). It's amazing to me how quickly this plant is progressing through the stages, especially that it didn't seem to be phased at all by all the trauma it's been through. You must really know what you're doing!
I really do think that formosas would be the perfect fit for where I think you want to go with this tank. I've never had problems with one jumping before, but it should be noted that their true behavioral beauty and uniqueness shines through best in a species tank (certainly not with larger quick and nippy fish) with plenty of cover for them to feel secure in. Though they're quite diminutive, they're the kind of nano fish that is always active doing something interesting but not in a dartlike manner; rather, they spend their time puttering around. they're not schoolers either, so each has its own individual plan of action to carry out. I think they're much more interesting in that sense than other nanos, as they seem to spend their time curiously exploring rather than in a perpetual state of nervous, twitchy paranoia... now, of course, that can be a good behavior as well, but I feel the behaviour of formosas would much better fit the vibe of this tank. Personally, I think their activities would emulate the swimming pattern and behavior of larger fish that would be found prowling the root systems of overgrown riverbanks with either unbelievable accuracy or a somewhat farcical leer that makes us laugh at ourselves for ever considering the two alike.
then again, I've only seen the thing empty with the beginnings of a bonsai in it so I really can't tell what you have planned!
here's a link to a video (not mine) demonstrating a little of what I was talking about. Planted Native Killifish tank, Heterandria Formosa in HD - YouTube
at times they actually remind me a little bit of pea puffers in certain movement-related behavior. (ie, they can swim backwards!) There are two main color morphs, the black barred/striped one (that would be striped and/or barred, they may exhibit only stripes or bands or both) and the 'gold morph', which is pale gold in color and sports little pattern. I hope this helps you decide, and I'll definitely keep an eye on this thread!