You should expect supplemental jurisdiction to "be inconsistent with the jurisdictional requirements of section 1332." There is no need to have it if you meet all of the easy requirement of § 1332 in the first place (i.e. you don't really care about supplemental jurisdiction unless you can't get diversity or federal question jurisdiction in the first place). The issue with your defendant is not domicile or citizenship, it is the amount in controversy. $25,000 is $50,000 too little for a suit to be brought on its own in federal court even if there is complete diversity. So, there is your inconsistency*.
The reason why the claim is bad has to do with the "by plantiffs" claus in § 1367(b)
§ 1367(b) wrote:In any civil action of which the district courts have original jurisdiction founded solely on section 1332 of this title ...
- Good so far. This is your $100,000 claim.
§ 1367(b) wrote:... the district courts shall not have supplemental jurisdiction under subsection (a) over claims by plaintiffs ...
- i.e. your NY residents
§ 1367(b) wrote:... against persons made parties under Rule 14, 19, 20, or 24 of the Federal Rules of Civil Procedure, ...
- i.e. bringing in the other defendant under § 24 because it arose under the same "case or controversy"
§ 1367(b) wrote:... when exercising supplemental jurisdiction over such claims would be inconsistent with the jurisdictional requirements of section 1332.
- because it is less than the required amount in controversy of $75,000
The reason for this is to give defendants added as parties protection; give them some say as to their forum. When someone files as a plaintiff, they have a choice of federal or state. Defendants mirror that choice in most instances because they can file to remove or remand to state court. But, people added as defendants by plaintiffs get stuck in federal court and they may not want be there over state court - defendants don't want to be in any court. There is no reciprocal rule for plaintiffs being added because no one can be compelled to sue as a plaintiff like they can be as a defendant.
I hope this helps
If this were not an inconsistency, then you could conceivably have 75,000 claims for $1, or just one $75,000 claim that involved one million people who each claim less than that much in their pleadings and the court would have unqualified § 1332 jurisdiction over them all - they don't!