It's true we do know that psions are not the same race as the cabal. However it is not known why they working together. One possibility is as you said, a slave race who the cabal conquered and then realized their potential. However they have psions flayers powerful enough to pull moons out of orbits, this suggest to me that they have the power to kill the cabal but choose not too which could indicate they are not forced into this. Another possibility is that they are similar to the covenant in Halo. They are two different races brought together by some event. Perhaps they have always been allies. Perhaps they once fought and realized in order to survive they must cooperate. We know the cabal are running, we have seen images (holograms) of what appears to be their home world which appears to have been severely damaged. The psions may be in the same situation. About the cabal blowing up planets and moons that they pass by. I do not believe this to be a evil behavior, but a combat tactic. They are running from something, armies would typically blaze the ground they lost upon retreating so that the advancing army cannot follow as easily. The cabal may well be doing the exact same but due to the massive scale of this retreat, planets are fields, moons are bunkers and we just happen to be their next stop.