I recommend the PS One dual shock pads (original Sony ones and NOT 3rd party pads), they look identical to the Dual Shock 2 pads, but use digital buttons (v.s. analog). Hacking one pair of these not only gives you access to PS2 games, but also (with converters) Gamecube, Xbox, and PC. There aren't any good ps2 to dreamcast converters, so hacking the dreamcast madcats you have is really best for the dreamcast. Use a common interface between your control panel and pad hacks so you can swap them out (I use 15pin molex connectors).
Happy hacking!