Slowing down judicial appointments because the POTUS is from the other party, and then speeding them up when the POTUS is in your party, is mostly certainly breaking with precedent. McConnell actually bragged about doing that on Hannity. Voting to drop the number of Senators to confirm a SC justice from 60 to 51 is most certainly breaking with precedent (and it was the GOP and McConnell that did that, the Democrats under Reid only lowered it to 51 for lower federal judges, and that was only after Senate Republicans refused to confirm many of Obama's nominees for the lower courts, leaving them empty.) Holding up Garland's nomination and then appointing a right-winger after you win the election is certainly breaking with precedent, as well as blatantly partisan for a supposedly non-partisan position. Doing a 180 on your statements about not naming a judge in an election year, then ramming through a new justice in record time during a global pandemic is certainly breaking with precedent, as well as blatantly hypocritical. So, yeah, Angus, the GOP has most definitely changed the rules for choosing Supreme Court nominees.
And for what it's worth, the Constitution says nothing about the specific size of the Supreme Court, nor does it restrict in any way expanding the Court.