Can sprites made in flash be in other fps than 24?
The character that you made is animated in 24 fps... can i change/make that or others animations for the game to a custom 12 fps?

Yes, you can. I've got different Flash sprites at different FPS in my project (though I've kept my main character at 24 FPS and just double frames to get an effective 12 FPS. This way when I want the character to have an animation that's faster or smoother than others, I can still pull it off.)