Can the "Two-Fisted Shooter" rogue feat give a Small creature the ability to use a crossbow?
You can treat the hand crossbow as an off-hand weapon, and you can reload it one-handed
The Hand Crossbow and the Crossbow are not the same weapon (both introduced in the PHB). Crossbows are always two handed weapons; Hand Crossbows are listed as one handed weapons, though they typically need a free hand to load due to the Load Free property. According to the Weapons and Size rules entry (RC 270), Small creatures can't use two handed weapons unless they have the Small property, such as the Shortbow.
The Hand Crossbow, being a one-handed weapon, can always be used by a Small creature. Two Fisted Shooter adds several benefits to it, but it doesn't change the usability. A regular Crossbow, being a two-handed weapon, can never be used by a Small creature, though Two Fisted Shooter does increase the weapon die size of a Hand Crossbow to that of a regular Crossbow.