If you use Bootstrap from the source, it's actually pretty modular. I bring it in with npm, then copy the bootstrap.less and variables.less into my own project (updating the references to those in the node_modules path)... this lets me do quite a bit in terms of not loading the kitchen sink, while still being flexible.
As an aside, I'm not quite sure why less hasn't won over sass... By nature of less being JS driven it is much closer to best in breed tools you need anyway for web development (npm) in my opinion. It kind of bugs me that to do modern web development, you are likely to want/need to have Node (or io.js), Python and Ruby installed. I tend to stick to Node based tools (there is sass for node, but it brings in a binary module). This isn't to be expressly negative, just that I'm surprised there's still broad support for both less and sass... with sass having a bit of an edge in the greater community, it would seem.