Chaos or not chaos?

I'm thinking of getting the shiny new starter box, with them chaos marines in it. Might be fun to have them fight alongside the orks, give them the three things orks don't have - firepower, manouverability, and survivability. It would also make sense story wise, since they probably coexist on the 'ulk. Or, well... coexist insofar as orks can coexist with anything really...