I'm not aware of any official regulations about that, but the only question I can answer is regarding whether there could be a conflict of interest between an owner's personal goals and what's best for the team - certainly that's possible imo. For example if Armstrong did hire Andy Schleck, but made himself captain instead of Andy that would not be the best decision for the team, since Schleck quite clearly is the better Grand Tour cyclist at the moment. The only thing that could prevent such a conflict is, that not all pro cyclists are complete idiots, and unless the money really is that good, might not sign with his team if they know beforehand that it's going to be like that.