Their "Just look at Galore/Pipp! They're clearly..." annoyed me, but I don't see anything wrong with genderbending (and/or trans) headcanons for cis characters. It's not the same as misgendering a real person. I mind when it starts to sound like gender essentialism ("I think this male character is a woman because he likes flowers/is nurturing/etc."), and in the same vein, you shouldn't gender- or cisbend(?) canonically trans characters. It's too close to what happens in real life.

Galore is an edge case, I think, because he is not not cis.