The only thing I want to add is that boas can contract minor diseases that are fatal to pythons. You might be able to "get away with" housing the two pythons together, but putting a whole 'nother genus of snake in with them is certain trouble.

And of course the obvious size difference. I have two females that are probably a half year apart in age. My ball is about 3 ft and 620g's. My boa is 6.5 ft and 5 pounds. At the VERY least my boa could easily crush my ball just by moving over her. In other words, she wouldn't even need to be trying to kill my ball to do it. It could be a total accident, but why would you want to take the chance at all?