I have a mono, two azuls and am about to add two kelberi and a xingu into an 800g.
It's been my plan for a year or so and the kelberi and xingu have been in a grow out attempting to get big enough not to be eaten by the others. The closer I get to the point of putting them all together the clearer it is to me that it's too much for that size tank. Add to the mix that they're pretty feisty and it doesn't make it any easier.