Thank-you for this very thoughtful reply, it's not unhelpful at all. Have you found that boisterous top-middle fish like rainbows can scare other fish? I find it very interesting because I found that when I added my cardinal tetras all of the other fish (apistos, otos and especially kuhlis) became much more bold and come out a lot more. I definitely don't want the rest of my fish to be spooked, I didn't realise that that was a possibility with something like a threadfin rainbow group. I also don't want any new fish to not be happy either! The first thing I'm doing before buying any new types of fish in this tank is to increase the numbers of what I already have to see what changes that brings about and what space is filled (or not filled), though atm nothing uses the top third which is why I was interested in hearing what people think, and if there was a fish that would use that area more.With your lovely layered planting that's going to get thicker, plus the apistogramma in the lower areas, you might not need to think of normal top dwelling fish. I'm a huge pencilfish fan but it's a real risk to keep them in an open top. The same goes for any top dwelling species as they tend to have evolved to leap when spooked rather than dive for cover.
I think that if you got a traditional middle swimming tetra or similar fish, like black phantom tetras, then the vegetation and the apisto's would push them more to the upper waters. I know tetras aren't as exciting as rainbowfish or killi's but they are harmonious and generally quiet, plus interesting when the males spar. My mindset for fish is perhaps different to others/most, I'm a less is more and like to keep things kinda similar in local, at least down to continent, plus I like quieter fish rather than those hyper active species which I find distracting (although great if activity is what you are after).
So my tip is to not be constrained by traditional categories of fish. Try to think what you want the species to bring into the tank, like if all your other fish are quiet then adding a really active species probably won't impress them much and you lose a lot of their behaviour. Bright and colourful doesn't always mean exciting, and common and bland isn't always boring.
I know that's probably the least helpful post when asking for fish ideas but from my time selling fish I know that often people had buyer regret when what they picked just didn't fit in with their tank or what they wanted to achieve. Consider less species but more of them. Your dwarf rainbows would be a nice colourful addition but even species like silver tip tetra can look and act amazing on mass.
I think I am a quirky fish keeper, I love a quirky fish with lots of personality, it's why the dwarf cichlids are my absolute favourite. It's also why I completely broke the same-continent rule early-on when I started fishkeeping last year - didn't understand stocking at all - and so got some kuhlis (the way they wiggle around is just so amusing), which rather doomed this tank to never match properly, so I'm not quite as bothered by it as long as the fish aren't upset by each other. I am planning a blackwater SA tank for my living room for next year though, properly kosher.
I wasn't the biggest fan of tetras in aquariums until very recently. They always look so incredible in films of them in the wild in enormous fabulous groups, (I am actually currently planning a to paint a mural in my hallway with huge schools of them), but in tanks alone they didn't appeal that much. However... I have found that my cardinals (from an acquaintance who was giving up the hobby) have really thrived in the new tank, are so much happier than in the temporary 60L and do form an interesting dynamic group, constantly making different shapes. I was planning on getting a few more of them, though they'll probably go and live in the blackwater to enjoy rooty brown heaven next year. And they do seem to make the other fish less shy, I was hoping that the rainbows would do that too, with fancy wiggly fins on top.
Won't get any pencils, I'll keep them on the list for a future covered tank! One day I WILL have some hockeysticks!! Another very amusing fish.