When it comes to the two Shinken Red keys, I don't think Toei wants that much of 'poking holes,' like looking too much into logic. I think its just
for fun. The point of the Hime Shinken Red I see realistically in Toei's
stand-point is just to show off the first female red ranger, regardless
that she has the same powers of her male counterpart. I don't think we
will ever see Gokai Red, Gokai Blue, Gokai Green or Gokai Silver use the Hime Shinken Red key. I think the
highlight of the keys is exactly whats going to happen in Ep
41---Shinken Red and Hime Shinken Red AT THE SAME TIME which we have
never seen before. I think that is the main reason they made the two keys. Because plenty of people have been asking since Hime Shinken Red had been revealed to have a key to what is the point if she has the same powers as the male Shinekn Red and that the female Gokaiger can use the male key.
Now as for gender changing the Extra Heroes, there is no way to gender change Rio, Mele,
Wolzard, Kuro Kishi, DekaMaster and DekaSwan. I can kind of see
Magimother and Signalman gender-changing but I don't think it'll happen. I don't see a change in Mele because she has a female figure, the breasts and various other things. A skirt on Rio would look silly. DekaMaster and DekaSwan fall under the thing that Dekarangers have no skirts. Wolzard kind of alreayd has a skirt of sorts so it would look weird. Kuro Kishi with a skirt would look strange, as GoseiKnight too, hence we never seen a female GoseiKnight. Even though their teammates (GingaPink, GoseiPink, GoseiYellow, respectively) have skirts, their suits are radically different that it could fall in the realm that they can't gender change.
Signalman has a wife Sigue that has an apron and earrings, so a female Signalman theoretically could happen but that would too fan-indulging I suppose, a fringe sector of the fandom.
And as for Zuuban, it goes with the
Dekaranger/Changeman/Bioman/Fiveman, etc. logic that if the females didn't have skirts
to begin with, then when they gender change, the suit wont have a skirt.
Since Zuuban has no team counterpart, it remains the same, also I see
Zuuban as genderless, it was just an it, not really a he or she. I don't
think he can reproduce. He sounded male, but again, poking holes. So I
don't think we will see gender-changing Extra Heroes because that is too
much hole-poking.
Now as for gender changing the Extra Heroes, there is no way to gender change Rio, Mele,
Wolzard, Kuro Kishi, DekaMaster and DekaSwan. I can kind of see
Magimother and Signalman gender-changing but I don't think it'll happen. I don't see a change in Mele because she has a female figure, the breasts and various other things. A skirt on Rio would look silly. DekaMaster and DekaSwan fall under the thing that Dekarangers have no skirts. Wolzard kind of alreayd has a skirt of sorts so it would look weird. Kuro Kishi with a skirt would look strange, as GoseiKnight too, hence we never seen a female GoseiKnight. Even though their teammates (GingaPink, GoseiPink, GoseiYellow, respectively) have skirts, their suits are radically different that it could fall in the realm that they can't gender change. 
