Fiendish Heritage roll #16: You have over-sized limbs, allowing you to use Large weapons without penalty.
Inappropriately Sized Firearms: You cannot make optimum use of a firearm that is not properly sized for you. A cumulative –2 penalty applies on attack rolls for each size category of difference between your size and the size of the firearm. If you are not proficient with the firearm, a –4 nonproficiency penalty also applies. The size of a firearm never affects how many hands you need to use to shoot it, the exception being siege firearms and Large or larger creatures. In most cases, a Large or larger creature can use a siege firearm as a two-handed firearm, but the creature takes a –4 penalty for using it this way because of its awkwardness.
So, my gut reaction is to shout "NO!", but do these interact at all? Is there particular reason to deny it besides the number of damage dice it'll deal?
For that matter, how about the large-handed tiefling and a Double Hackbut?
This double-length rifle uses a pair of trunnions to mount its barrel into a swiveling mechanism fastened to a lightweight, two-wheeled carriage.
It takes a full-round action to set up the carriage. The carriage has a hind leg, allowing the wielder to wheel the device about and immediately prop it for stability during combat. Unlike other two-handed firearms, you must fire the double hackbut while it is mounted, or else firing it imparts a –4 penalty on attack rolls and the recoil knocks the wielder prone. A Large or larger creature can fire a double hackbut one size smaller than it is without its mounting as a normal two-handed weapon and without the danger of being knocked prone, but takes the normal penalty for firing an inappropriately sized weapon.
In any case, I'd like to clarify how the "large creature + siege firearm" works. As it says 'Use as a two handed firearm', is it safe to assume it'd take all the standard rules for using a two handed firearm, just using the siege cannon as the weapon stats?