Note on Supplemental Jurisdiction and Complete Diversity
Although there is some conflict in the caselaw, the prevailing view is that supplemental jurisdiction under section 1367 does not extend to non-diverse defendants or otherwise operate to defeate the "complete diversity" requirement of section 1332. See annotation notes 52 and 53 to 28 U.S.C.A. sec. 1367.
Given the uncertainty about interpretation, you should be able to make textual and policy arguments on both sides of the issue. In formulating arguments, you should consider the effect of Exxon Mobil.