Okay, so I know the Titan Fighter is allowed to use one size category two handed weapons with a -4 penalty, but my question is are they allowed to use 1 Handed weapons of a size category two sizes larger since a creature one size larger can wield it as a two handed weapon? Same question for a Light weapon 3 size categories higher.
no.
"a titan fighter can wield two-handed melee weapons intended for creatures one size category larger than himself"
So since a huge 1handed wasn't meant for a large creature a medium titan fighter can't wield it.