RE: What are the next best commands?
The heel command was the second command MOJO learned following the first which was sit. There is no trick. Just takes lots of time and long walks. Use a choke collar. Your dog will relize that by not pulling the chain the pressure is released from his neck. Every time his head is even with your calf give him the "heel" command. In my earlier post I talked about "Water Dog" which is a wonderful book or video/DVD and also attached a website for all the text you should probably need to train a waterdog.
You can teach multiple commands. The dog needs reputition. Do not teach commands that you notice your dog is confused by. I taught sit, heel, and extended the sit command for stay all at the same time. MOJO had no problem with it. Never heard anyone say that there was a magic number for the limit of commands. I would guess it could be a lot. Just depends on how much time you have.