I am relatively flexible about a plot where someone trains as a Sith but realises their ways are wrong and gets out before it is too late.
But the idea that you could be actually fighting for the Sith, advancing their evil aims, but be on the Light Side... and the other Sith are ok with this? At this point I am querying why you are using the term 'Sith' at all, as it is pretty much contrary to all presented meanings of the term, film and EU, that a. a Light Sider would fight for them and b. the Dark Lord(s) of the Sith would be ok with having a Light Sider in their ranks.
We all know that doesn't really scan on so many levels.
I mean, the Empire are INVADING the Republic, yes? For purely selfish purposes? With a surprise attack based on a pretence of peace? And I know the Republic has its corruption issues, but they are still the injured party here and the Jedi are defending them not because that's just what side their on, but because it is the right thing to do. How many have died because of the invasion, born out of selfishness and greed- the essence of the Dark Side?
So why would anyone on the Light Side fight FOR that? And considering the enormous possibility that they would realise "hang on, I'm fighting for the bad guys here, shouldn't I stop them instead?", why would their commanders not have thrown them out (or, more Sith-like, killed them) ages ago?
This is getting a certain distance from some basic Star Wars principles. If you want to create a political organisation which has both Light and Dark Siders fighting for it (geez, that's already not very Star Wars) then whatever, but to then try and pass off such a thing as 'The Sith', which is a pure bad guy thing in continuity... seems very weird to me.