Good suggestions and nice start on the reference.
The only thing that I would add is it's probably a good idea to have the sculpts sized the same as Rainman's (especially the diameter of the necks) so they work well with RM's outfits. If these end up being an improvement over his sculpts, then there will be a lot of people looking to upgrade those sets but it would be pointless if the sculpts don't jive perfectly with the outfits they already own.
NO, not the same as Rainman's. Too big. They need to be slightly smaller.
These are CK's, not Rainman's, so make them as CK's. The heads will still go with Rainman's outfits i'm sure, but why should that even be a consideration...?