So the problem is that a lot of rhythm game controllers can’t just be dual-modded with a normal pad – they present themselves to the console differently. On PS2 I think you can get away with it, but to get everything working perfectly you might have to resort to scrapping a second peripheral (say, a broken drumset from the other console). It really depends on the software, but AFAIK it’s pretty tricky/damn near impossible to make a normal controller get accepted for playing guitar/drums on 360/PS3. PS3 would likely be possible with a custom firmware for whatever dual-modding board you’re using, though, so it could just take the extra step of changing the USB descriptors. But I’m no expert on it, and haven’t really looked into it myself.
Dance pads, though, yeah. They’re easy. Most nicer pads that I’ve seen (including the one I have) actually come with a DB-15-based breakout box so you just have to figure out the pinout and it doesn’t even take internal modification to make work!