Volume format (HFS or HFS+) is irrelevant on any volume shared via file sharing. it's always the local machine the disk is attached to that handles file reads and writes and therefore that's the only system that cares.
When using something like FireWire Target Disk mode to directly attach a drive to a system, HFS+ will work on any recent (8.6+? 9.0+? I forget which) OS. Since you're probably not using Target Disk Mode with the Performa, this probably isn't an issue.
In short, the performa will be able to read/write to any shared disk attached to the iMac.
However, I wouldn't follow your thinking on partition scheme. There's really no/little need to have separate partitions for Mac OS 9 and X. I'd go with one big partition for the whole lot, or maybe one small partition for an emergency startup volume.