I can't really talk to your concerns since it is just my husband and I and my 2 dogs. However, if it is something that you are wanting to do, and you are prepared to invest the time and energy in the new dog then I would say - go for it.
One thing you would need to ensure is that the new dog was getting consistent messages from everyone in the house - this would be very important. So, I would recommend a training strategy that everyone is aware of and agrees to - so it doesn't really matter who is home, but everyone is treating the dog consistantly.
The one positive about getting another dog now is that Otis is still young enough to want to play - he may be a little more grumpy with a new dog when he is 9.
By the way, i would definately recommend getting a female next. A male and a female are definately the best combination to have. Also, I would pay particular attention to the dog you are getting and make sure that the personality will fit in well with your current household (you wouldn't want an overly dominant dog, or one that is overly timid either).
Good luck with whatever you decide!