I don't use completions with sufficient panache to answer this fully, but:
In .sublime-completions files you can define the scope. In theory, you should be able to expand the scope of the CSS completions to also include SCSS, LESS, etc. Unfortunately, the default CSS completions are coded in a plugin not in a completions file. (This is quite unusual. IIRC, only the CSS & HTML completions work this way.)
I had brief look at the completions script and you would need to tinker with lines 148, 156, 158 and 186. They look like regular scope selectors, so you should be able to delimit multiple scopes with commas... At least that's my guess.
Let me know how it goes