When I worked at the Blue Cross, we used Petplan 6 week insurance (this meant that it covered the puppy for 6 weeks after it went to it's first real home) and the insurance was not valid until the puppy was 8 weeks old. We had to fill out quite a complicated form with the new owners, and all the puppies had to have a full health check prior ro their re-homing. If it transpired that they had a condition which could affect the insurance then that had to be clearly stated on the forms, otherwise the whole thing could potentially be declared invalid by the Insurer. Some things, such as a bout of conjunctivitis or antibiotics needing to be given for any other little problems also had to be stated, even though they may not subsequently have any adverse effect on the level of coverage ....
I agree that 6 weeks is too young to be re-homed or separated from Mother or litter - there are also so many potential problems that may not manifest themselves during those two very formative weeks..
From the point of view of monitoring weight, and doing primary worming and vaccinating, I think 6 weeks is very young for a puppy to go to a home. Mind you, it depends upon the home you can provide and upon where they are coming from - I am thinking that in the best case, they are able to spend another 2 weeks becoming socialised and having the comfort and support of their litter mates and their Mother - obviously there are some sad cases where these things are either not possible or are neglected ...
I would say that if you do take on a very young puppy, the onus is on you to constantly monitor its health, in that you need to do daily weight checks, stick to the socialization regime very carefully, monitor their diet and hydration very carefully and basically take care of them as if they were almost like hand-reared babies .... Ensure that they are wormed with proper puppy preparations and that they are microchipped and registered with the appropriate database.
Good luck, and keep us informed - with a backup like all the puppy-specialists we have on COL you are bound to be fine !!