What are the rules in custom builds?

So I was at a hobby shop today and saw a guy playing Tau. A large number of his miniatures were clearly 3D printed or altered versions of other minis that he was using for something else. One example that stuck out was a regular tau scout that he had glued a sword to and said was his “ Cadre Fireblade”. It clearly was not. When I asked if he was allowed to use those as a fireblade ( I was just an observer) he got super defensive and mad and started going off about how it’s fine and allowed and he’s not giving GW a penny more, and all this. His tone made it clear he receives similar lines of question frequently.

He seemed like he was not in a mood to answer my questions, so I am here. Are you allowed to do this? I had an opponent once tell me I could not use my legit GW Cadre Fireblade because the little corner of the cape has busted off and the cape fell off and got lost. That was the actual miniature itself and I was told I can not use it due to missing the cape but you’re allowed to just give a scout a sword and you’re good without having to spend $32 on the actual cadre? And also the 3D print things. I was told if it’s not entirely GW, it is illegal for use. How can these people have full armies of 3D printed trash.

Another one that I saw was he had three stealth battle suits that he put on bigger bases and glued a gun next to and was playing them all as broadside battle suits instead despite looking very different in both appearance and size. What do you think? Whats allowed? What is the actual official rule? What are most peoples’ general opinion on this?