To the best of my knowledge, you can't add to the list of what mw.loader.load or mw.loader.using support, but you can still use $.getScript() without loading anything extra, since jQuery itself comes as part of the default Mediawiki install. If you have cascading dependencies, such as a script that makes use of a $.getScript()-loaded plugin, simply nest each level of dependency inside the $.getScript().complete() callbacks just as you would with the callback you'd give mw.loader.using.
As a side note, using $.getScript to load javascript on pages outside of Common.js also allows you to use ES6 in those non-Common.js scripts. (Mediawiki still doesn't support ES6 on Common.js.)
For a live example, look at lines ~306-335 in https://stratics.com/w/index.php?title=MediaWiki:Common.js