sounds like the consensus is that a brand name in a sub domain is ok. You asked about companies sueing you, well the first step a company will take is a cease and desist letter from their lawyer. Basically they will tell you to stop and shut the site down. Then you can decide if you want to continue and take your chance or just cut your losses and move on. In a legal and ethical way, at the minimum, go about your business until someone tells you to stop.