Between the levels I will be working, I could work any of the deep three spots, and will also be working some referee. I don't think I want to by 4 SS and 4 LS shirts! I would be fine with buying a short and long for R, and a short and long for S, but to buy 4 more for a possible game or two at F or B is kind of silly. I'm sure they will have something to take care of the pocket also for guys who work multiple positions.
In our area, it does not matter what letter you have on your back for the frosh games.
I have not bought new shirts in a few years and have a similar predicament that you do. I bought a F, B & S in both long and short, so six in all. When I R or work LOS at the frosh/reserves, I’ll just wear what I have.