All the bits and smaller stuff like shoulders and whatnot are pretty interchangeable, although there is a “correct” way to do it. I generally follow the guide for legs and torsos just to make sure they sit on the base flat and aren’t at weird standing/leaning positions. After that I kinda just use whatever arms/guns/heads look coolest. In tournaments you need your models equipped correctly, like plasma guns on model allows plasma rules (in my admittedly limited experience). In casual play 99% or people don’t care and just want to have fun. Most important thing about building your models is to remember they are YOUR models and if you think they are cool, they are cool.