I find this an interesting debate.
I also think the Twili are not the Gerudo. The only arguments that support the Gerudo=Twili theory are the red hair and the fact that Zant has a Gerudo symbol on his outfit.
I think the red hair aren't that convincing. Have you seen the change from Zora to Ruto? There is only one conclusion you can draw from such a dramatic change: that all Zelda theory with the word "evolution" in it, has no value, since there isn't any logic in Zelda games regarding "evolution"
The Gerudo symbol on Zants outfit is probably just there because he got his magic from Ganondorf. Someone countered this argument by pointing out that Zant was wearing the same outfit during the scene in which he met Ganondorf, which is logically speaking a solid point. However, I don't think Nintendo found this little detail important enough to alter his outfit for that scene.
The main reason I believe the Twili are not the Gerudo are the following:
- Midna stated several times that Zant was using a "different kind of magic", which he turned out to have gotten from Ganondorf. So however Ganondorf was banned to the Twilight Realm (in an impulsive act from the sages) he still was 'different' from the other Twili.
- Not only was his magic different, also his appearance is completely different from the Twili. So the only confirmed-to-be Gerudo in the game doesn't even slightly resemble Midnas true form or any other Twili.
I think these two points are solid enough to debunk the whole Twili = Gerodu theory.
Perhaps after Skyward Sword will be released we will see that the whole 'Ghirahim Tribe'/dark interlopers makes a striking resemblance to the Twili and conclude that the Twili actually were the dark interlopers. We'll have to wait.