Too many questions.
First of all, your SDCard is
16 Gigabytes - if you have that full of stuff, it ain't all from just apps, I guarantee you that. You've either copied stuff on there, or else have been rooted for a while and made numerous full system backups that are eating away your space (currently, my backups run 500-600 MB in size - each).
As for what can be removed and what cannot, we can't play armchair quarterback here - I could tell you to delete the folder, let's say,
com.espn.score_center - but that would be good advice only if you no longer have ESPN ScoreCenter installed. If you still have it installed, then tell you to delete it would effectively be telling you to waste your time, b/c that will either 1) break the app, or 2) be recreated the next time it performs an update of scores / you run the app.
So, in effect, you're asking us a very impossible question to answer, as we do not have direct access to your phone.
As for adding apps - umm, apps, by default, install to your phone memory, not the SDCard, unless you're running an alternate ROM and / or are rooted and have enabled the Apps2SD script of some sort to force all apps to the SDCard. If you fall into the latter category, then you really need to read this:
App Install Location | Android Developers
Finally, as for themes for Liquid's FY - the exact same way you got the original ROM installed.