Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
The basis is that it's a Grass-type pretending to be a bug, as a funny reversal of real-life bugs pretending to be plants (like leaf insects and walking sticks). So it has nothing to with not wanting to give it the Bug-typing out of competitive considerations or something. It's a deliberate...
It's not the sword itself that has a gender, but the ghost who has possessed it. Most other Ghost-types have genders as well, with only a few exceptions like Rotom anad Giratina.
Pachirisu is also in the Field egg group. Plusle and Minun are the only Electric-types who are exclusively in the Fairy egg group.
The others that are exclusive to the Fairy egg group are Clefairy/Jigglypuff/Chansey and their evos, and Audino.
Don't be too quick to overlook Fairy-type Arceus (for which we're likely to get a new Sylph Plate or something like that) and its Fairy-type Judgment attack.
Also there are multiple characters from fighting games who are called Goro, like the ones from Mortal Kombat and King of Fighters. So it is quite fitting for a Fighting-type Pokémon.
People said the same about the Ruby/Sapphire box mascots as well, but then they gave Groudon Drought to counter Kyogre... So they may give Yveltal an ability that gives it an edge (or even just a fighting chance) against Xerneas as well.
It has happened before, actually. The Rotom formes changed their secondary types between gen IV and V. So you might even say it has happened 5 times over already. Ergo, there definitely is a precedent for this sort of thing. I wouldn't be surprised if a couple of the older mons had one of their...