Hi and welcome to YorkieTalk. Your babies sound adorable.
Yes, I would definitely seek a vet who is well versed with smaller dogs. My Hot Rod had luxating patella surgery on his one knee last year. He went from a grade 1 (lowest) to a grade 3 in a few months time. (Grade 4 is the worst-but lp can be worse). Just because your little ones knee is loose now, doesn't necessarily mean that it will be like that when he grows up. All puppies tend to have loose knees...Maggie had loose knees at her 11 week well check and she is fine at 1 1/2 years old.
You can also try glucosamine type of products. I would discuss this with a vet who knows about this as to whether to start these at such a young age however. You can also try getting stairs and ramps for them to keep them from having to jump too much. Jumping can definitely aggravate lp.
As for the humping, as lil missy say, it is a dominance thing. It sounds like this little one may be the alpha. Hot Rod was neutered at 6 1/2 mos and he has never marked in the house but does so outside. He didn't even really lift his leg to pee until after his neuter. We have keep them on pads in the house and he squats when using the pad but will lift his leg outside.
As for the fighting/biting, I am sure it is a puppy thing. Just correct with with a stern no and definitely make sure to get them some chew things. This will help them to learn what can be nibbled and what cannot.
Good dental practice is very important in dogs, especially little ones. You should start brushing their teeth early on to get them used to it. For now, you can use your finger and make the motions to get them used to it. Then I would recommend getting a soft toothbrush (I use a childs) and pet toothpaste. I haven't really kept up as I should but I try to do their teeth at least 2-3 times a week. I really need to try and do it daily.
Best of luck to you and enjoy your little ones.!
Can't wait to see pictures.