I make heads that exceed my 128x128 limit. I try them on by
making them a tga file and pulling them in my custom file. (While keeping
my current head in a safe folder) While I do this, is there a way that this
is against the rules? Or does the head not show up in ingame matches if it is
bigger than your limit? (Only show up in sp?)

i dont think your supposed to put it in your custom file, do the following.

goto forum.toribash.com
click "Shop"
click textures
now equip

u can do whatever u want with u're custom folder

Ya that what Missuse said.
It only shows in sp matches and only you can see it unless you actually wear it on yer actual head item.

Yeah, it's not bannable at all, or even an offence, you can do whatever you want with your custom.