The campaign is taking place a few months after The Sacking of Coruscant and the signing of the Treaty of Coruscant which split the Republic in half, between themselves and the Sith Empire. This is about 3-4,000 years before the Movies but still in the Republic days. The Infinity Empire existed some 20,000 years before this game's timeline 
I am not 100% on grey knights... you can play it as a philosophy if you want to but the Game Mechanics is another thing. Having and using Dark Side powers gets you closer to the Dark Side and the only way to lose those points is by spending Force Points. It can be done but you would have to be very sparing in your usage AND not be doing evil deeds at the same time (i.e. using Force Grip to chock someone to torture them for info, etc.)

I am not 100% on grey knights... you can play it as a philosophy if you want to but the Game Mechanics is another thing. Having and using Dark Side powers gets you closer to the Dark Side and the only way to lose those points is by spending Force Points. It can be done but you would have to be very sparing in your usage AND not be doing evil deeds at the same time (i.e. using Force Grip to chock someone to torture them for info, etc.)