To answer your direct question, no Two Weapon Fighting does not apply to rifles and heavy weapons. Two Weapon Fighting eliminates the attack penalty imposed when you have an off hand attack. Since rifles and heavy weapons do not have an off hand attack, there is no effect from Two Weapon Fighting.
However, you should give Canderous Two Weapon Fighting, because he is far more effective with two melee weapons than any rifle or heavy weapon in the game. The only reason to ever use heavy weapons is if you have the downloadable content (Yavin Station). Even then, Canderous is better served dual wielding the swords available at Yavin Station. If you don't have the dowloadable content, dual wielding Bacca's Ceremonial Blade (fully upgraded) and the Genoharadan Poison Blade is far more effective for Canderous than any ranged weapon.