you shouldn't have any problems fitting those controls on that panel, however it could be a little tight if you have 2 adults trying to play at the same time. i tried making my first panel 23" wide so it would fit inside the cabinet, however after a few mockups, i decided to go ahead and cut the sides and build my own box. i'm very glad I did. i made the new panel 12"x30" and there is plenty of room for two large adults to play.
download the mameroom control panel designer software, you can set the custom dimensions of your panel, and then add components and move them around to see if they fit. it is a great tool, and it is what i used to design my panels.
http://www.mameroom.com/files/CPD_Setup.zip i played around with it quickly and you should have no problems at all getting a 6 button street fighter layout on that size panel. just figure that you need 1.5" between the centers of the button holes, and i like to have 3 inches between the center of the joystick hole, adn the center of the closest button holes. also, don't forget that the graphics representing the joysticks don't account for any wiring that may stick out of the sides, i made this mistake, and put my joystick too close to the edge of my panel, and i had to use a router adn route out a space on the side of my cabinet for the wiring for the one switch...if you are going with standard happ super sticks, allow for about 3" from the center of the hole to any sides so you will be safe.