1. To answer your first question, I would ride it until at wears out. It's pretty obvious when it needs to be replaced. Since you're already questioning whether to change it I would go ahead and do it because the last thing you want to think about while riding is your tires. You should be focusing more on the traffic or your riding line.
2. I go through 2 rear tires before I go through a front so I always keep the same brand rear as the front. If you change the rear to a different brand or style of tire you may lose a little bit of the handling characteristics you're used to. For instance, I run Pirelli Rosso's () but am thinking of going to the Michelin Power Pilots (). I'm going to wait until I need my front again so I can change the whole set at once.
3. I'll let you in on a little secret also. Hopefully when you need to change your front you still have a little meat on your rear so you can use it as a burnout tire and then get your new set.
:D:D:D