Would it be against the rules for me to (attempt to) add a scammer flag since I personally haven't dealt with the user in question?
Yes. You can only leave the weakest-type flag if you weren't harmed personally.
Just to confirm, you are not allowed to create a contract violation flag unless you were personally harmed, correct?
Correct.
I understand what Theymos is trying to do with the flag system but not being able to an obvious scammer, and supplying supporting evidence, who caused people to lose money is odd.
EDIT: Can we try to not derail this topic with personal accusations? Please let's keep this about the flags in general.