First off...welcome to the board. Glad to have you.
Second off...NBA isn't basketball, it's entertainment.
OK, maybe it is a form of basketball, but you certainly don't want to watch it and form any rules interprutations.
A friendly pick up game with the guys should be just that...friendly.
Where I play, if a guy thinks he got fouled then he calls it. If the contact dosen't effect the outcome of the desired play, then he might "let it go".
Heck, many times a pick up game without an official go smoother than with a couple guys blowing those annoying things around their neck.
In other words, when a guy gets fouled he knows it...he dosen't miss it like an official might, or he won't pass on the call if he thinks he could have finished the play without the contact.
Having said all that...your basic question seems to be, "Can a player displace another player on the court because he dosen't have '40 more pounds of muscle' to keep from being displaced"? The answer is NO. That little guy deserves a spot on the court. If he is behind you and crawling all over your back or kneeing you in the A#@...call the foul. I know you just want to "back him up" because you are bigger, but that is not legal either. He deserves that spot on the floor, you can not legally move him without fouling him. BTW, if you have the ball in that situation it would be called a "player control" foul...many times the guys at the gym simply call it an "offensive foul".
A quick answer to your quick question about "ducking" your shoulder "to get underneath the defenders arm"...sorry dude, again, if the defender is set you cannot displace him by ducking your shoulder or any other means. However, that is a great move for a football player to make when wanting to avoid being stopped. (Somehow, I think you probably have played a little football in your time)
Hopefully these observations will help...keep on playing and keep on asking, it can only improve your game.
Dude