Just an interesting tidbit I noticed about Photoshop 6.
Since I upgraded to Photoshop 6 I noticed that you can have quite a few 3rd party plugins without upsetting the filter folder layout.
You can store your 3rd party plugins in a seperate folder outside the plugin folder for Photoshop 6. (I named mine 3rd Party Plugins) Then by clicking on Edit~Preferences~Plugins and Scratch disks when Photoshop is running, check Additional Plug-Ins Directory and then choose the new directory.
Close out Photoshop and restart and you will notice your standard filters plus your new directory will be available to you. There is still a limit but I haven't reached it yet and I have quite a few filters added. I was actually amazed at how many filters it does hold. I know 5.5 couldn't handle all my extras but so far 6 does.