I would agree with that. You want to be shown how to do a hook ... uppercuts are a little more self-explanatory, but still, having someone show you the correct way right off the bat is of immeasurable value.
Turn into your right hand much more - a guy like you, with the size you have, will have brutal KO power if you learn to, as we say, "sit down on your punches."
Oh yea. I can move a 70 like crazy, and I'm only 165 lbs. Typically, you want the bag to be about half your bodyweight ... so you, at nearly 300 lbs., really can barely get by with anything less than 125 lbs.
I understand what you're saying with the leg kicks. If it's a necessary evil to have it at the height, then go for it. I always forget about kicks and shit.