"Suzi is the starting point for all of our web projects and a culmination of 6+ years' experience in maintaining a front-end framework." - This wasn't built in a weekend and isn't for small projects. There are a lot of powerful tools and methodologies in this framework that should be reviewed if not also used for your next web app.
What I do when I see new Sass mixin libraries is I go through the library, find the mixins that I don't already use on a regular basis and save them to my personal libs. That way, I have my own library with the mixins I use the most.
Per usual, A List Apart delivers with an awesome breakdown of good and bad Sass practices and how to fix those bad behaviors not only for yourself, but for you team. Includes 6 tenants your Sass'ing should live by.
"With the addition of HSL to most major browsers and preprocessors, designers and devlopers can start to use a system that makes much more sense. Especially if you don't want to switch over to a visual color picker all the time (which is how I work with HEX and RGB)."
Learning from the big players who run large web apps and multiple marketing sites can be incredibly helpful for tying down your own architecture and build strategies. Here's Evernote's blog post on it.