Were those rule changes dome by Pius 12? And are they for the entire world or just USA?
Some were universal, and some were left to the US bishops, so the Dimond calendar is US-focused. There are probably certain feasts also that are celebrated in the US (at least at different priorities) than elsewhere in the world and vice versa. But I could see where one would accept the fasting/abstinence rule changes while using the pre-1955 Liturgy.
That's the problem with these calendars is that there's an admixture of Liturgical calendar and fasting/abstinence rules that could in theory be mixed and matched ... depending.
SSPX have even more complications because in theory they accept the Conciliar Church's major relaxation of fasting/abstinence and even Holy Days. To give one example coming up, for the Conciliar Church, January 1 is not a Holy Day of obligation, so to be consistent the SSPX can't really treat it as such (whether on their Calendars or otherwise) since if you accept the V2 papal claimants as legitimate popes, it's perfectly within their powers to lift the obligation for January 1.