To answer your first question, yes that is exactly where you should put your super-specific styles. You want to put your communal styles in your placeholder, styles that all types have but are different in the mixin (background color, etc) and the rest on the class itself.
The best way to get better at Sass is to follow people who are active in the community (which unfortunately is not me). Start with Chris Coyier and Hampton Catlin and work your way from there.
If you don't mind me plugging my own talks, I have a few that are really good at learning Sass:
https://speakerdeck.com/liamdanger/why-your-sass-is-bad-and-...
https://speakerdeck.com/liamdanger/what-if-css-was-object-or...