A few years ago (1970's and earlier) you were told not to bother starting ANY training until your puppy was 6 months old
You can imagine what hooligans must have been around then!!! Thankfully nowadays folk have realised that Puppy's education starts the minute you bring them home....or if you are really lucky the breeder will have started a lot of it already
I'm sure your wee man will soon be an absolute angel
As to weight - best to ask others with more experience on here
I've never had a yorkie pup so got no experience of this aspect (my one yorkie, Scamp, as a child came to us at 6 months old).