I'm trying to make an action that will change all fields to Mixed Case, but I want it to ignore "T.I." when it's found, how can I do this?

Here's my custom action:

Filed: TITLE ARTIST ALBUM GENRE ALBUMARTIST UNSYNCEDLYRICS
Case conversion: Mixed Case

and when I click it from teh action menu, it doesn't work at all.

How did you manage to enter so many fields for that action? I think you should only enter one field at a time .... and then create a separate action for the next field.

For the conversion you can specify which characters determine a word border. You would have to enter the full stop to get "T.I." capitalized.
The function itself does not "leave" anything "out" but only looks at the characters entered for "word starts after" and capitalizes the following letter.

I saw the list of commands and put them all in the same box, in the order they appeared in the interface, now that I think about it, that explains why only one command would show up, I'll make a combined action, one for each thanks, not sure why I didn't realize it earlier.

Is there anyway that I can set some files names to automatically be FULL CAPS, and Mixed Case for the rest?

Hardly anything goes automatically in Mp3tag. It is always up to the user to select the files first and then apply the modifications.
You may create several action groups that lead to different results.
So one action group results in all caps, another in mixed case...