Custom font characters

So I need 12 characters in the font that I can replace with custom characters of my own. What are some that won't be missed?

I'm looking at FF3usME and there's a bunch of unused, blank characters -- from index 0x60 to 0x7D, particularly -- can I just change those? If yes, how do I place them in the dialogue text later? Since there won't be any keys assigned to them, like a,b,c etc.

I've kinda done this before -- editing regular characters and using them -- but since I need 12 of them, well... that might be trickier to pull. Any ideas?