Great. Thanks for the additional information. I really appreciate it.
It's a sad fact that some dogs are simply born to be barkers. When you factor in the fact your dogs are all small breeds (known for being vocal) and there's a pack and you've got the perfect gold-medal barker.
As I'm sure you already know, yelling at the dog to be quiet won't work because dogs who are already barking, just interpret our yelling as an attempt to join the conversation (for example, if the dog is barking at the mailman, he's saying, "Hey! Owner! Come see who's outside!! Is this a friend? Is it an enemy?? What should I be doing right now?? Oh, you're barking loud too, so I should probably keep barking!!!"). Any yelling we do can accidentally encourage them to keep barking.
We're going to need to train him to act differently when he's barking.
When your dogs are barking, you need to ignore him. Wait for him to be quiet, and after a few seconds of silence, start using a key-work like "hush" and then giving him a treat. Make sure you do this immediately...timing is super important, but make sure you don't give him a treat until he's been silent for a few seconds. Try thinking like your dog...would you rather keep making noise, or be quiet and get a yummy treat?
Do this training several times daily until your dog has figured out that the key word means it's time to be quiet, and that being silent brings treats, but that making noise doesn't get him anything. Over time, you're going to make the time between when he's barking and his quiet longer and longer. After a few weeks of training, you should be able to use the key word when he's actually barking and then giving him the reward when he stops. You can invite family/friends and even the mailman to help with training by asking them to come and knock on the door. My mailman has helped me train many dogs.
It's important to remember to not repeat the key word over and over, since this can actually encourage him to keep barking rather than being quiet.
I also like using a back-up plan: diversion. Instead of using the 'hush' cue when he's barking, you can call him over to you and ask him to perform a desired trick like sitting or fetching a toy. Obviously, you don't want him to act like a mute if someone were to be bothering your house...it's good that he's letting you know someone's around...but by making sure to reward his being quiet, rather than his barking, you'll end up with a better behaved dog, and less stress for you worrying about his incessant noise making.
And please, please, PLEASE remember that this behavior didn't start overnight, and as such, you won't be able to break him of this habit in a day or two. Patience (and maybe a pair of ear-plugs) here is the key!
I hope this helps.