We're talking about throwing insults here, not legal definitions. Calling someone "plebe" does carry implication of inferiority, but going as far to use "serf", means underlining not only inferiority, but insult by implying near slave status of being owned "by us nobles". Imagine, just before secession war in US, a southerner saying to northern free afroamerican "dirty slaves should know their place", it would be something similar.
Anyway, Ranma's status as a rebelious and powerful human summoned by youngest, underage daugter of powerful aristocrat, and a daughter that did not prove her status as mage, is completly outside any form, legal or otherwise.
Anyway, Ranma's status as a rebelious and powerful human summoned by youngest, underage daugter of powerful aristocrat, and a daughter that did not prove her status as mage, is completly outside any form, legal or otherwise.