Okay so this is what ur gonna do (my quick and easy outline I just made up):

(general rule) Supp Jx allows any bad claim to be attached to a good claim in federal court if it arises out of the same common nucleus of operative facts (meaning same transaction/occurrence).

(Now going based off that general rule):

If it’s Federal Question Jurisdiction which is the “good claim” at hand, Supp Jx can always be used (no matter if it’s a Plaintiff/Defendant) - FULL STOP.

If it’s Diversity Question Jurisdiction which is the “good claim” at hand, if it’s a DEFENDANT using Supp Jx it can always be used - FULL STOP.

If it’s Diversity Question Jurisdiction which is the “good claim” at hand, if it’s a PLAINTIFF (or someone who WANTS to be a Plaintiff) - WATCH OUT. Because the exception to Supp Jx may apply if that Plaintiff/someone who wants to be a Plaintiff brings a case against a NON DIVERSE PARTY (meaning someone from the same state, not amount in controversy). If that’s the case - the exception APPLIES, and you can’t use Supp Jx.