With all the "inapropiate" indications that gem fussions mean I think that "the super gem fussion" is kinda "wrong", but if you concider that Sugilite look so monstruous then is probable that Rose was the one who keep that fussion in synchro and beautiful...
There isn’t anything inappropriate about fusion. I think either ruby or sapphire is iffy about amethyst and when three gems are fused at once, it’s not gonna be pretty. I think the mega fusion that is the temple was the way it was because they all agreed that the war needed to end. It was that they all had the same goals