The new instruction Universae Ecclesiae has this important paragraph.
28 – Praeterea, cum sane de lege speciali agitur, quoad materiam propriam, Litterae Apostolicae Summorum Pontificum derogant omnibus legibus liturgicis, sacrorum rituum propriis, exinde ab anno 1962 promulgatis, et cum rubricis librorum liturgicorum anni 1962 non congruentibus. … 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.
Derogate means that things are partially replaced, set aside. So, insofar as the use of the 1962 books is concerned, if there is something that came into law after 1962, and that thing or practice conflicts with what is in the 1962 books, then those post-1962 things don’t apply to the use of the 1962 books.
As I read this, and I checked this with canonists, since the employment of females substituting for Instituted Acolytes came with an interpretation of the 1983 Code, you cannot have altar girls for the Extraordinary Form which was, in 1962, carried out by all male ministers and servers. This would probably apply to other issues, such as the substitution of music, the use of proper vestments and choir dress, who gives which blessings, etc.