The Instruction on the application of the provisions of Summorum Pontificum, entitled Universae Ecclesiae, in paragraph 28, says in an English version:
Furthermore, by virtue of its character of special law, within its own area, the Motu Proprio Summorum Pontificum derogates from those provisions of law, connected with the sacred Rites, promulgated from 1962 onwards and incompatible with the rubrics of the liturgical books in effect in 1962.
So, if there is a conflict of law between what is found in the 1962 books and what was issued subsequent to the 1962, the what was issued subsequent does not apply for the celebration of the Extraordinary Form. That would include such post-Conciliar innovations as female service at the altar.
UE 28 clarified that females are not permitted to act as servers for the Extraordinary Form.
You may recall that at the University of Cambridge in England, even before Universae Ecclesiae was issued, the Catholic chaplain, who had also celebrated there the older form of Mass, made a decision that his male, female integrated team of servers, should serve also at the older form of Mass. That meant that he employed altar girls, women, to serve at the Extraordinary Form. When Universae Ecclesiae was issued, he continued with this practice.
This situation generated claims on both sides regarding UE 28, some people claiming that that paragraph did not bar females from serving.
I have now a copy of a letter sent by the Pontifical Commission “Ecclesiae Dei” and signed by the Secretary Msgr. Guido Pozzo, which clarifies that UE 28 in fact does say that females cannot serve at the altar in the Extraordinary Form.
The letter essentially quotes UE 28 and then states:
In this regard, the Circular Letter of the Congregation for Divine Worship and Discipline of the Sacraments of 1994 (cf. Notitiae 30  333-335) permitted female altar servers, does not apply to the Extraordinary Form.
Bottom line: females may not serve at the altar in the Extraordinary Form.
However, I understand as well that, though the chaplain at Cambridge has also had this clarification from the PCED for some time, as recently as Saturday 5 June he continued to have females serve at the altar for the Extraordinary Form.
This all goes beyond the single issue of the sex of the person serving at the altar. There are other innovations subsequent to 1962 to which UE 28 applies. It is important that, early on, the meaning of UE 28 be clear and also be accepted. It is important that, early on, the PCED get involved when Summorum Pontificum and Universae Ecclesiae are misinterpreted.