So a couple things:
Original Problem
Gulp-order is definitely not necessary. You can easily do
"main.css": {
"files": ["styles/bootstrap.scss", "styles/material-design-bootstrap.less", "styles/main.scss"]
}
To get the same result. The order in the array is the order they get concatenated.
Unhelpful Suggestion
In your situation I would just use Less. Go ahead and ditch Sass for this project. I personally prefer Sass by a longshot but I am not married to any preprocessor really. It all depends on what the project requirements are. If the project hinges on that important Less dependency I would just use Less and be done with it.