I think there is a problem with the way you have structured the concept. Plurality, as you use it, is usually defined by most online dictionaries in political terms
the number of votes that a politician or party gets in an election that is more than any other but is less than an absolute majority
It is determined based on two disjunctive characteristics:
- largest single group, but
- not a majority
The smallest minority is just that - the smallest group. There is not another characteristic that makes it analogous to plurality.
In fact, the term minority itself poses a problem. It is generally defined in the context of two groups
the smaller in number of two groups constituting a whole; specifically: a group having less than the number of votes necessary for control
Further, the term minority can encompass more than one group when there are more than three categories. Which of those (or which combination of those) is of interest depends on the context of the categorization. In social situations, there are often a number of minorities identified based on a variety of characteristics differing from the majority. Further, a number of political or social minorities may coalesce to form a plurality.