I'll name what I couldn't keep mine with rather than what I could. I couldn't keep it with anything that constantly swam back and forth at a fast pace, I tried bala sharks of equal size in separate occasions and he longest it lasted was maybe a month or two before he decided to maul them. I've not had luck with silver dollars either, but this may be due to size, they were to tall to be eaten, but were short in length compared to him, he bit them in half, he'd eat the half in his mouth and leave the other half and just bite another, for whatever reason. I've only tried to keep him with jaguar cichlids and this did not work either, they challenged him and it didn't go well for them. I couldn't keep him with other hoplias either, they're known to be aggressive to other hoplias though. On the other hand I have my odoe and gold wolf who swim all day but not at a fast pace so he doesn't bother with them much, however sometimes he gives a little chase but they're faster than him, if they weren't, I'm sure I wouldn't have an odoe.
So with my specimen anyway, I can't keep anything that swims back and forth at a fast pace constantly, can't keep anything that will push the curu to the edge and not take the warning bites and learn their lesson. There are exceptions to this. My odoe is fast enough to outrun him and big enough to fight back, which sometimes she chooses to do. Or a wyckii, who made it through the bad beating the wolf gave out after constantly pestering the wolf.