I wouldn't worry about the welts and scratches unless they bleed. This breed has a high pain tolerance and a scratch isn't going to mean anything to them while they're playing. That's why you have to keep a close eye on them while they play. That isn't to say you should let them scratch each other the heck up, but one or two scratches isn't something I would get concerned about.
For the walks, it depends on whether they walk well on the leash and you think you could reasonably control them should something happen unexpectedly (like a squirrel running across the sidewalk, a loose dog running up to them).
6 months was about the age I started walking my two separately, but I'm 100lbs soaking wet, so your mileage may vary.
Treats are very good and, imo, necessary. You don't work for free, so why should your pups?
In the beginning, when they're just getting the hang of the command, give them a treat every time they do it right. Then, as they get better at the command, start to drop the treats off so they don't get dependent on SEEING a treat before listening to you. So ... they do a sit correctly 10 times in a row, a treat every time. Then you start only giving them a treat when they do a really fast sit. Or give them a treat every other time, then every other two times, then every other three times, etc.
