Do the penalties for the Feat Two Weapon Fighting stop at -2/-2 while holding a one-handed weapon in your primary hand and a light one in your off-hand? What about while dual wielding light weapons like Rogues do with the Archetype Knife Master? Would the penalties be negated?
Light Main-hand weapons do not reduce the penalty.
Two-Weapon Fighting (TWF) does not care if you have a light weapon in your main hand. As always, it is hard to prove a negative, but as you can see, the rules about TWF make no adjustments for reducing it below -2/-2 with Feats. As you can see from simple Feat searches, they have not added anything that is not accounted for in the original TWF rules.
There are other ways to reduce the penalties. One way to further reduce the penalty is the (relatively new) Dual-Balanced weapon modification. Note that if you do not have the Weapon Adept feat, Modified weapons are one category of training higher than normal (ie a dual-balanced dagger is a martial weapon). As well, the Duelist's Vambraces can reduce the off-hand penalty by (up to) 2 for one attack.
Some subsystems include ways to reduce or negate the penalty, notably Mythic and Combat Stamina. See the TWF feat itself for those details.
A Light Weapon Doesn't Matter
No, using a light weapon for the purposes of Two-Weapon Fighting only matters in the off-hand. Using a light weapon in the main hand doesn't change the penalties for attacks with either weapon.
Reasons to Use Two Light Weapons
Although using a light weapon in the main hand doesn't change the TWF penalties, it can help in other ways.
First and foremost, if you're using the same weapon in both hands, any benefits you get from using that weapon will apply to both hands. The most obvious cases are weapon-specific feats like Weapon Focus (shortsword) and Weapon Specialization (shortsword). Classes that specialize in particular weapons can get even larger benefits. For example, the unchained rogue with Finesse Training, or the warpriest with Sacred Weapon.
There are also some small reasons to use light weapons in each hand, even if they're different weapons. The Piranha Strike feat works like Power Attack, but only for light weapons. Also, if a creature swallows you whole, you can only cut your way out with a light weapon.
The Shield Master Feat
If you use a shield bashing build, the Shield Master feat eliminates the TWF penalties for attacks with a shield. It also lets you use the shield's defensive enchantments as if they were weapon enchantments, adding to attack and damage rolls.