This isn't an official study, but I've observed many times when working cattle that two or three in a pen will be calm and easy to handle, but isolate one of them and it's a whole different story. I'm with SoB on this one. Not saying that one won't be okay by herself, but I'm convinced they'll be more content with others.
As for the OP's question about adding a bull, adding a young one will likely be fine, but as he gets older he's likely to not be satisfied with just one cow and start testing fences. Especially if there are other cattle nearby. I don't know if that's the case here.