However, if you are in a rush or find yourself wanting to spend your life with him, get out before you have real problems. Plus, I've worked for friends in a similar situation and it ended in breakups and an attempted suicide. I wouldn't do that, but neither do I want to be part of it again because I know how ugly it can get. He could be useful, but you hire the scut help, not make them partners. I just do not want to sink the ship over some stupid guy.
Nothing can break up a relationship faster than going into business together.
Obviously you and your partner haven't worked out the rough spots in working together. You need to know who's in charge of what, and how you're going to work out problems that come up. In the case of the ex-boyfriend as a partner, of course you're right. It's a very bad idea. But that's the least of your problems.