I think it's not about which side themselves acts on collective versus individual motives, but whether those motives are applied internally or externally. You could correctly identify the Jedi as "collectivists" who teach a very strict set of rules and guidelines for their members. But they do not project that outward onto other people. Those rules apply to only themselves and their own members.
The Sith, on the other hand, employ "individualist" motivations, but they also project what they want/do on others. I guess the point here is that it doesn't matter how you live your own life, or even how a group live their own lives. What may make one "evil" is the degree to which they force others to comply with their own ideology and rules. The Sith have a desire to rule others, while the Jedi do not. To me, that's the more important distinction.
Uh... Meet the new boss, same as the old boss? I think the issue of slavery on some worlds in the galaxy is tangential to whether the Republic or Empire was good/evil in broad terms.
I will point out, however, that the very same aspect of the Jedi that resulted in them *not* running around rooting out slavery wherever it existed, also meant that they weren't running around imposing their own morality on the galaxy in other ways either. It's really easy to say "make everyone not do something we don't like", when (almost) everyone agrees about disliking that one thing. Where it gets tricky is that usually, once some group is empowered (or even just feels justified) to eliminate the "one thing everone agrees needs to be eliiminated", they rarely stop there. There will always be some "next worst thing" that needs to be adusted, fixed, changed, whatever. The size of "(almost) everyone" shrinks, but the desire to fix/change things remains strong anyway.
Which will inevitably lead to authoritarianism. Even with the best of intentions. Which is why the Jedi don't do this. The Sith, on the other hand, do not have "best of intentions" to begin with. So whether they allow or stop slavery has nothing to do with their own moral judgement of slavery, but whether it helps or hinders their own objectives. If allowing the Hutts to engage in slavery helps the Empire, they'll allow it. If it hurts them or is actively being used to raise wealth for dissidents or trouble makes, they will crush it. There is no grand moral/ethical consideration here.
For the Republic, it was less about "do we like this?", and more the nature of a Repulbic itself. Which, I suppose, we could correlate somewhat with the Jedi philosophy as well. It's about allowing each member world to rule themselves. The positive of that is less authoritarian government. The negative is that some worlds will have laws or rules that other members wont like. It's one of the great paradoxes of government power. The more capable/empowered it is of removing "bad things", the more likely it is to become that "bad thing". Recall that it was exactly the inability of the Republic to "impose order" (on the Trade Federation, and other factions), that lead directly to the Empire.