Find your natural slot and see what breaking ball comes off of it instead of trying to make a breaking ball fit a certain slot. Anytime you change a slot you lose your fastball slot. Typically people are 3/4 slots but they think they are over the top...over the top is rare...so in the 12/6 breaking ball. A slurve is more common and a good pitch for what it is...dont force yourself to be at a different slot just to fit a breaking ball...breaking balls are way overrated.
Screwball is not something you need to worry about...just like was said above...focus on 2 seamer and change up. The reason we always try and create different pitches is for confusion to the batter...but if you can't control your fastball or change up what is the point?
You develop arm strength by throwing, anything that is manufactured such as devices to help you throw harder is false. You have to know how to throw and be comfortable throwing.
Knuckleball is a good pitch to have in your repertoire if it is something you usually throw. But you have to buy into either being a knuckleball pitcher or a pitcher who throws a knuckleball. Pitchers who throw a knuckleball is seen as a gimmick instead of the other way around where you are a knuckleball pitcher.