We could easily argue, though, that those people were there solely for training purposes - and that in normal circumstances, nobody would even enter the facility where coveralls appear to be required, and all engineering functions would be handled from a shirtsleeves environment similar to the TOS facility.
In TMP they would in turn be there because this was the first time the engines were being used.
Sure, we could argue that "progress" has taken place between TOS movies and TNG. But TOS already showed that Scotty tended to work pretty much alone, and very seldom got really
down and dirty with any repair job. He was one of the few characters never to wear the wraparound coveralls, IIRC!