(1) Yes, the Stacks (capital ‘S’) plugin allows you to use stacks (lower case ‘s’) elements in your web pages and project, but you still need to select a theme to work inside. Whether you use an open design like Foundation, Freestacks, Pure, Bootstrap or Blank… you still need a canvas within which to place your page elements (stacks).
(2) I understand that the Foundation stacks are optimised for use with the Foundation framework - which the Foundation theme itself delivers. You can certainly use non Foundation stacks in the Foundation theme at the probable cost of those optimisations. Not sure whether Foundation stacks would work in other themes (I bet some of them probably would) but I’m sure a Foundation fan (there are plenty here!) will chime in with more specific details.