Hi, the standard prodos became prodos8 with the IIGS, while prodos16 is at the core of GSOS only.
so any floppy which is prodos-formatted and with basic.system on it, would do - you have to prodos-format a 3.5" disk to have it boot, and put basic.system on it. In fact, whatever .system file is at the top of the catalog list, will be booted once prodos has loaded.