Both Uses of

inferior
in
The Idiot

In the American War some of the most advanced Liberals took sides with the planters on the score that the blacks were an inferior race to the whites, and that might was the right of the white race.†
Chpt 2.10

inferior = (adjective) of low quality, or of lower quality or rank than something else OR (more rarely as a noun) a person of lower rank or status

He accounted them immeasurably his inferiors, and it was this feeling which caused his special amiability and delightful ease and grace towards them.†
Chpt 4.6 *

inferiors = People who are of lower rank or status

Definitions:

(1) (inferior)
of low quality, or of lower quality or rank than something else -- sometimes used as a noun to refer to a person of lower rank or status
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

Less commonly, specialized senses include:
- anatomy and botany -- a lower position on a body or plant (or pointed downward)
- typography -- a character that drops beneath a printing line (such as with subscript)
- astronomy -- having an orbit relative to the sun that is lower than earth's (making Mercury and Venus inferior planets)