It's styli, Skyblazer, and you're right that styli fit only in cartridges from the same manufacturer. (Exception: Radio Shack carries, or used to carry, styli for several different makes of cartridges, mostly Shure if memory serves, but the quality is definitely entry level.) As for my cartridge history, the last one before the Grado was an Audio Technica (sp?) of about the same price level. I still have it but I think the Grado sounds a bit better with the music I like, which is mostly classical. The differences are not dramatic.
The February Absolute Sound has an article about the latest model of the London (formarly Decca) cartridge, of a unique design (you'd have to read it), which I'd like to hear sometime, but the thing costs almost as much as five 990s and a complete record player - turntable, arm, and appropriate preamplifier - of commensurate quality for this little gadget the size of the end of your pinky would probably cost around twenty thousand dollars.
Kind of out of the range of us 990 owners.
Back in the real world, if you want to replace your Pioneer cartridge / stylus, good ones to look at would be Grado, Ortofon, or Sumiko but there are plenty of others.