With a
USB adapter and
joytokey you should be able to use a gamecube controller, but Stick is great for big hands, I have sausage fingers and the big buttons are easy to use.
Actually, if you're using the Mayflash Gamecube adapter, you don't need any software. It runs just fine; Skullgirls not only detects the adapter and all 4 of it's controller ports, but even properly names them, too. Gonna tag
@Quid as well since you were replying to them.
The big problem with controllers on PC is whether or not the game supports the proper input device APIs. The most common (and as far as I'm aware, only two video game controller APIs used in modern games) are Dinput and Xinput. Dinput is an older interface, but for most controllers, it works fine. Xinput was made by Microsoft to make the Xbox 360 controllers work; without Xinput, the analog triggers wouldn't work, since making them analog technically makes them a third joystick, and Dinput doesn't understand what to do with that to make it function properly as a button.
Problems arise when for whatever reason, game developers prioritize support of one API over another, or leave out support for one entirely. Skullgirls has support for both. The SFV Betas on PC have no Dinput support, and nobody is sure yet if they will fix it in the retail version. Third party software like x360ce or InputMapper work by tricking Xinput into thinking that a Dinput compatible controller is a 360 controller, but this can be problematic for a bunch of reasons and isn't a viable solution.
That being said, if the game you're playing on PC has Dinput support, then almost every controller outside of the default Xbox 360 controller should function properly. If the game has Xinput support, then all of the Microsoft compatible peripherals should work fine. So it really just comes down to how the game is made.
Keep in mind that I'm not a programmer, and it's possible that I have some of my details incorrect; but to my understanding, this is how things works.