I tried (and in the meantime suceeded) to install a dual boot system with NT4.0 and RedHat-6.1. To do this I used the NT harddisk manager to create a number of partitions that I wanted to use under Linux. NT labeled these partitions as DOS FAT16. Later, during RH6.1 installation, I was unable to use the DiskDruid to change the partition types to Linux and Linux swap. I had to resort to the command line fdisk program to do this.
Disk Druid will not allow you to change many details about existing partitions, only the mount point basically. This is a safety measure so that people do not change the partition types of stuff that already existed when they started the installer. You would have had to delete the partitions you are referring to and then recreate them in Disk Druid in order to change their type. This is not a bug therefore.
In my experience Winxx is very picky about partitions. Modifying the partition table behind Windows' back is almost always a recipe for desaster. In order to be a replacement for fdisk during the install process DiskDruid should be at least as powerful as fdisk.
Disk Druid is meant to be a safe replacement for fdisk, not a complete replacement. This is the reason that it makes certain assumptions, like not changing the ID of existing partitions and requiring swap partitions to be created. If you would like more power than this, then fdisk is available to you.