How much is too much?

The only thing i can tell you is when the angels start to pair off you will have problems i had 8 angels in my 150G and when they started to pair off they just about killed the other ones and i had to get them out of there. I think if you really wanted to you could get away with adding a few more tetras.

And the balas will out grow the tank as well. Plus when they aren't kept in larger groups they tend to get really skittish and often end up killing themselves by ramming in tank walls. They can get well over a foot long. A better "shark" choice would be a red tailed or rainbow. They like to be the only one of their kind and stay a bit smaller.