Puppies taken at six weeks miss out on a socialization period meant to be spent with mother and litter mates. This is where they are supposed to learn many of their dog on dog language and interaction. Things like bite inhibition and appropriate play are developed here.
Honestly, I can't say exactly what they learn from this period besides social behavior, but I can tell you as a trainer, I see the other side of this much too often. Puppies get taken at six weeks and come to me some months later for obedience training. They have a tendency to have terrible bite inhibition, lower than average self-control, trouble settling or having an "off-switch", trouble interacting clearly with other dogs which tends to lead to misunderstandings in body language, and just strange behavioral quirks that I usually don't see in ones taken at 8 weeks. I've found that neurotic behaviors are much more common in ones taken younger.
I've had puppies taken at six weeks that came to me later and were compulsive tail-chasers and paw chewers and light chasers. I have had others that had hangups about certain physical interactions no matter how hard we worked on counter conditioning and desensitizing. One that absolutely would not let you hold her feet for more than 10 seconds. We finally got to ten seconds after about four months of work. Mom did all the proper puppy handling protocol, but puppy just never liked having paws handled where others taken at 8 weeks and handled were just fine.
Best thing would be for him to stay with mother and litter the extra two weeks. If that simply is not possible, then make sure you have well-controlled play times with well-adjusted, socialized older dogs (like your own) and if you know the other owners of the pups, getting them together for puppy play times could also be helpful. Be VERY clear about personal and environmental boundaries - start bite inhibition early. Start working on attention and self-control games. Teaching a simple watch me, take it, wait, and leave it will help with that. Work on settling in the house and being quiet and calm with you. Don't reward any crazy over-excitable behavior. It's fine to be excited and maybe barking when running around outside, but literally bouncing off the walls of the house is unacceptable. And make sure you are unyielding about these rules. There CANNOT be moments where the puppy is just too cute and we're going to let him slide just this one time. It'll come back to bite you. Probably literally. :p
Even if you get him at six weeks, I think he'll be alright as long as you put in the work. A large part of it is accepting that getting hi so young means you need to substitute that part of his socialization and it takes a lot of work and a lot of planning. But you're getting good information on the forum, so I think you're well on your way to handling it! :)
EDIT: If he is taken earlier, he will also have a much more people-oriented socialization which means people will probably be the source of a lot of excitement and he'll be quicker to practice those bad people behaviors like jumping up, chewing ankles, and barking for attention. I would jump on top of these as well and work on keeping attention and calm around strangers as well.