To answer some of the questions I'm seeing here: First, the IIgs is compatible with nearly every Apple or Macintosh floppy drives. The only exceptions are for PowerBooks or USB. Personally, I rotate (since I have more computers than I have working drives) a 5.25 in, and a 3.5 in Unidisk, as well as an "800k drive" with a 1.44 MB drive (extracted from my LC), installed into the case. I even rigged up a 1.44 MB from a Quadra once, and it worked fine (although I never got it to fit into an external case). My IIgs is an unmodified, bare-bones ROM 01 (256k, no extras), and so far it formats and reads any Apple II disk I put into it. Mostly I use the 5.25 since they're the most compatible with the rest of my Apple II line, and 800k is getting increasingly hard to find. Eventually, I hope to get a Floppy EMU.
Second, you can move programs from a 5.25 in disk over to a 3.5 in. So long as there is a suitable boot program or operating system on the disk, the game will work. This goes for the //e as well, as my grandfather used to do this all the time. However, my //c has not been terribly friendly with 1.44 MB.
And third. Yes, the IIgs will use the same 9 pin joysticks, and paddles as a //e, //c or Mac LC with //e card. I rotate my single joystick between those machines all the time. However, the joysticks and paddles for the ][ or ][+ are 16 pin and sadly will not work, but they will work with a //e. Also, I don't know, but I'd love to find out if an ADB joystick would work?
Finally, I've got to try Arkanoid!