29 gals is generally considered the minimum for a single angel, to my knowledge, so I would have to disagree with that. I wouldn't put one in a 20H, or in this tank that is a bit smaller.
It is just a little off from the standard 20H over here, so that does really open up your tetra options. You would not be limited to just embers and neons. You could do a school of flames, glowlights, head and tail lights, rummynoses, green fires, or cardinals to name a few. I see you did pick up neons but, just wanted to let you know anyway.
I think the high mortality rate in the oto's is in large part due to improper stocking and supplementing. I haven't had problems with my oto's at all, but it is crucial to wait until the tank has matured and not overstock them (three would be perfect for your tank). If you do decide to give them a try down the road, just be sure to supplement with blanched zucchini and/or shelled peas weekly in addition to a regular algae wafer supplement. They are often picky eaters that don't eat the wafers. This is the same treatment I would recommend and use myself for all algae eaters.
If you are looking for something else that would fit in nicely I would recommend marble hatchets. I keep them in a 20 gal with a betta and some tetras and it works out very nicely. The tetras swim low and the hatchets swim high and the betta is all over the place so it fills out the tank well. Just an idea.