What Withers sells
Hirelings unlock at character level 3 and cost 100 gold each. Up to three can be active. A full party has to swap someone out. Dismissing one sends them back, and recruiting them again returns a level 1 copy that keeps its equipment. The Outsourcing achievement is "Recruit a hireling."
Respec of a companion also costs 100 gold. It returns that character to level 1. Race, background, and appearance stay.
Origin companions are the story
A hireling is a class you field. An origin companion is a person with camp scenes. The twelve hirelings sit on the uninfected side of that line.
The romance overview's partners are the origin companions Astarion, Gale, Karlach, Lae'zel, Shadowheart, and Wyll, plus Halsin and Minthara. Jaheira and Minsc are named there as people you cannot romance.
Halsin, Jaheira, and the twelve hirelings are not infected. They cannot use illithid powers.
The level cap still applies
The character level cap is 12. Withers is the respec: class and ability scores change, and the character starts again at level 1.
Official site: baldursgate3.game.