I wanted to use Bootstrap 3 and Less together. Here is what I did:
If you used
..inside your main less file, then the bootstrap.less will be included during compiletime and will be inside your compiled styles.css.
After your styles.css has been compiled, you only need to include this file into your project.
PS. Also take a look inside your styles.css file, to see what has been compiled inside there. Or play around yourself/experiment. For example, create 2 different less files, @import them inside your main.less, compile and see what happens.