So I got the symbol changed for the CSS but in battle and on the results screen it's still the default symbol.. how do I change that as well (where is the file I need to change)
Those are stored in different files, you need to change those as well.
Do you know where those files are?
The files that contain the ingame HUD are inside the info2 folder (you can get them from the resources section)
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Open one file (for example info_boss_battle_en) using brawlbox, expand MiscData[30], then expand the textures folder and look for the symbol you want to replace (you can replace character names too, the textures are inside the same folder ;) ). After finishing save it and do the same for every other file inside info2 folder.
Info2 folder goes inside the pf folder.
As for result screen icons, I don't know how to replace them :P
I believe the results screen icons are models with no textures. However, people have made custom ones (that are textures) for new slots with BrawlEx.