I don't know how any of these species would do in your tank, given its shape, but if you're looking for smaller alternatives to Balas, maybe another "shark" or a catfish?
Red Tailed Black Sharks and Rainbow Sharks have the shape, though you wouldn't be able to keep a group like you would with balas due to their temperament.
Denison Barbs, or "Roseline Sharks" have that shape as well, and are a bit closer in color to balas, but are much more docile than the RTBS and Rainbow Shark. They are another torpedo shaped fish, though, and would likely prefer more horizontal space.
As far as Catfish, there are the typical Corydoras, but I think a lot of Synodontis catfish have a fairly similar look to balas. I've never kept any myself, but I believe they tend to stick towards the bottom, so they wouldn't provide a bunch of movement in the upper levels of the tank like a group of bala sharks would. I am personally a fan of
Synodontis petricola.
Actually, an even better substitute would be the
Pictus catfish. They have a very similar look, are kept in schools, and I believe tend to be out and about more than other cats, but I've never kept any so I couldn't say for sure. The only issue with them is that they aren't that much of a jump down in size - much more reasonable the balas, but they may need more horizontal space than you can provide. I would probably look into these more, if you're interested in them.
You may also want to look into different species of barbs. My searches so far have turned up similar, smaller fish, though not as small as you would probably need.
Keep in mind with all these suggestions that I'm not sure how well they'd do in a corner tank or how they do with plants. But they are things worth considering and looking into more if you're looking for a good bala substitue.
