Personally, I would not tempt fate by asking a barky dog to just bark once then be quiet. I would get a better doorbell and ask him for silence. (Also, whilst the quiet tip is used a lot, I genuinely have never really seen it trained to much effect, except in dogs who were naturally inclined to quiet down after a few barks.)
Changing the doorbell also has the benefit of it being a totally new sound - much easier for him to learn to do something different to a new sound then to change what he does to the old one.
Then the answer is treats 
The doorbell rings, you immediately toss very tasty treat in his bed. Ideally, someone keeps doing that all the time the door is open and stops as soon as it's closed again.
Before long, he hears the bell and runs to his bed for the treat rather than bothering to bark. Keep that up, at least intermittently, for a very long time until it is totally ingrained behaviour.
Strangers in the house is a bit different. For this, you need to be clear what is causing the barking:
a) fear - prioritise keeping everyone safe and give him somewhere to treat to out of the way
b) excitement - work on him being on a lead and only getting to say hello once he has quietened and calmed down