Relative pronouns (like quis/quis/quid doesn't) don't agree with its antecedent --their antecedents for case; instead, it agrees withgets that from its position in the subordinate clause. Note that they still agree in gender and number, since it's supposed to be a stand-in for the noun.
Because the pronoun stands in for "servi"a nominative use, the subject of the clause -- i.e. "servi"servi non parent" -- it has to be nominative plural, orwhich means it's "qui". Because "servos" is plural and masculine, the pronoun does as well.
You can find out more about relative pronouns here.