First you have to change your mindset. YOu don't really have to try to become the leader of the pack, mainly because there is no pack. What you have is an interspecific assemblage of animals. These pups aren't fighting you for leadership and if they do something it's not out of some machiavellian motive. You can choose to view the dogs as your enemies constantly trying to take over or you can view them as potential partners that need to be educated.
I would suggest you start walking them, traning them, even cuddling them individually. Who would want to spend time with a stranger that doesn't speak their language? You need to make your company far more rewarding than the environment and other dogs - this will means food rewards and play. Also, I think having them spend time separated from each other, and you would be a positve experience. This will make your entrance into their world so much more rewarding. After all, if they are together hanging out, you don't have much to offer.
If you walk them alone and it decides to stop, then let him. You job as this juncture is to make the walks and the leash a positive experience. Just stand there and wait.. after all it's a pup and it could be tired, it's thin pads may hurt from teh pavement or salt or it just wants to smell the surroundings. It doesn't really matter. He's with you, bonding with you and having a positive experience with you and that's the really important part - aside from seeing and experiencing new things - of the walk at this age.