You know...ember tetras, celestial pearl danios or white clouds could work in there perfectly. A school of 6 of any of those species would be fine (heck, for the embers and celestials....you could easily do 8 or even 10). We're talking 1" fish here. Ratio-wise, that's much better than an oscar in a 125 gallon tank or a convict in a 55 gallon, based upon fish size to tank size.
In an established tank, I think 6 otos would be fine in there. Also, 6-8 Corydoras hastatus would be fine in there as well. They're much more diurnal and school in the mid-water column.
There are more and more tiny fish coming into the hobby these days, and many of them would do good with shrimp.