Hello Alan, welcome to the forum.
The toon characters here that Apparition has created, are based on the G5 standard...meaning they work with head/upper/lower/hands/shoes. The clothing is a cloth made in 3dsMax. These characters as you see them would not be possible without that program and Apparition is a teacher and has been using the program for years. I simply do the clothing/hair and some accessories. I also use Zbrush.
The beauty of this method, is that the characters are 100% compatible with all standard iClone motions with the added benefit of being able to use the avatar controller to animate the face parts. And they can wear most any G5 cloth system. The 4 characters that come with the Toon Kit do have some clothing options (i.e. material file changes), but their heads cannot really be used on other characters as they don't fit and if you add other G5 cloth systems to them, it changes the body underneath the clothes to that of the cloth you are adding. Therefore it changes the shape and distorts the body. These Apparition characters maintain their body shape and texture.
I don't think there is an easy way to create characters this way. Apparition has spent hours in MAX reshaping the bodies of these characters in ways that would not be possible inside iClone. I have been experimenting though, and what you can do is added these heads to Jimmy Toon or Mr. Pose...Jimmy Toon for sure can be exported to zbrush and reshaped...with Mr. Pose, you would probably need the export license. Here is a pic of Mr. Pose Round using one of the heads. One limitation though of reshaping the bodies is that you cannot change the poly count. So if you export the "upper"...you can reshape it, etc. - but if you add any polys it will not let you replace the mesh in 3dxchange.
Vidi, another fine developer, is creating her character from scratch and she uses 3dcoat and then rigs the character in DAZ. This allows her to bring a dressed character into iClone and allows for some very unique body and character design...she is posting pictures of her work in progress. This type of character is fixed (non-standard) though but if you have the time and skill, it is certainly possible.
Warlord creates characters in Zbrush (non-standard also) and rigs them and brings them in through 3dxchange.
I'm certainly not the most knowledgeable person around here when it comes to character creation so if anyone else has anything to add here, please feel free.
"Wise men speak because they have something to say; fools because they have to say something"