Is pair when used as a subject always plural?

No, the noun pair is a singular noun that can be the subject of a sentence or a clause. The noun pair can also be the object of a verb or a preposition. The plural form is pairs. Examples:

subject: A pair of shoes was left on the back steps.

direct object: She wore two pairs of socks because the boots were too big.

object of a preposition: He is one of a pair of twins.