Puppylover, I don't want to rub you the wrong way and this is, of course JMO, but I think you are being a little naive. In no way, shape, or form should a 6 month shep/lab mix and a 7 month whippet/lab mix be left unattended, loose in a back yard, for 8 hours a day. And don't tell me that you have eyes in the back of your head that you KNOW what they do when no one is there to watch them. I can only say that if you have not had any issues so far, you have been lucky, but odds are, your luck will run out. I hope not, for their sake and yours.
Before having rottweilers, my ex husband and I had a yellow and a black lab, a golden, and a chessie/lab cross. (My ex was an active bird hunter.) I know from experience that labs are prone to joint issues just like shephards (yours being a lab/shep mix), so IMO your 6 month old pup should have supervised exercise and playtime. At least until he is 12 months of age, even 18 months. I always like to error on the side of caution. With the experience you say that you have, you should know this. It takes nothing to tear a cruciate (sp?) tendon or pop a shoulder in a young pup.
Again, not trying to send you on the defensive, but wouldn't it be wiser to be safe than sorry. Just something to think about.
As far as the weight thing, if your vet says that they are fine then I wouldn't worry about what anyone else thinks. They didn't go to school for 12 years for nothing. I think a vet can tell if your pup is in a healthy weight or not. I personally keep my rotties on the trimmer side, more of a working weight, but that is just my preference. I believe that a trimmer rottie puppy is better. You never want them to grow too fast. HD, ED, pano, are all effected by weight.