That is correct. Lemmesee now...You need the base Trim-pro, you need the case holders for the specific cartridge or family of cartridges (260, 7-08, 308 would use the same one, since they are the same base cartridge case), so you'd need one for the 204. The trim pro 3-way cutter head works for most all cartridges (maybe not for the 50 BMG, however), so you only need the one cutterhead and you'll need the .20 caliber pilot for the cutter head. And, of course you'll need the case holder and pilots for any other calibers you want to load for.
The only very minor irritant is that when I change cartridge cases (from 223 to 260 for instance), I have to change the cutterhead pilot, which is very easy, but then I have to make minor adjustments to the cutters to get the deburring and chamfering just right. It isn't difficult in the least, but it takes a few minutes to set them to suit me. I could just buy multiple cutterheads, one for each caliber, but haven't done that yet....but I might.
There are plenty of other trimmers out there, and one or two might be better than the RCBS, but I'm quite happy with the Trim-Pro. Might even be better to have the powered one, but I don't feel that I need that upgrade.