Few things, first the giant danios will probably be eaten when the vieja is full grown. I tried it once with a vieja in a 125 and they disappeared. Also, I think the blackbelt is going to be much more aggressive than the other two and probably tear them up at some point. I would call a 125 about minimum for a blackbelt, it may not want to share. If you really wanted to try some other cichlids with it, might have more luck with some convicts or maybe neetroplus, a couple pairs of them. But the large vieja still may kill them, it would be a risk.
Better idea for a community in the 125, IMO, would be to pick less aggressive fish. Since you like firemouths, do a group of 6, then also could try vieja heterospila, much less aggressive.
