Not an issue on the sides, look at the photos in centurion submissions. Thing to remember is the kit is "one size fits all", but you have to make it fit you - if that is trimming or adding shims. You are doing the testing correct with always wearing the undersuit.
For the shoulders, this is what I did:
Get the ab and back together. Then attach the back plate. Placing the front to the ab so the spacing is correct (the reveal), then I could see how the tabs between the two needed to be cut. You will have to adjust and cut them. I had to heat bend mine to get them to lay over, then I worked on the amount on each side to cut.
In this photo, I was not wearing my undersuit (bad TK!), everything was taped so it was always moving around. Make sure to use gaffers tape if you can, that will help. The back piece was not pulled all the way up (as I didn't know I would have to heat bend the tabs to better fit).
Keep up the good work and ask questions, we need more troopers for the empire!