Hi!
Here are 2 tests, based on your demos:
http://smartcore.ru/test/ - with FA icons
http://smartcore.ru/test/index2.htm - with UI icons

1. You can easily change size of the base text font or FA icons font via Firebug/Developer tools to check how it looks in different situations.
2. I deleted all lines about modal dialog buttons from your FA integrations CSS.
3. JS needs to be changed to make different button structure (see my original comment in grid.js Oleg's FontAwesome plugin openpsa/jsgrid#71 (comment)). In this demo JS isn't changed, so there is no space between text and icon.