Yes, you may lose a mb or two due to partitioning, because partitions can only start and end at certain points on a hard drive. However, it's not too much to worry about.
I'm not sure if the Redhat (or Fedora Core) installer can resize NTFS, so you'll need to get a live or installer CD which has the necessary tools. I'd recommend
Gnoppix, which is just a retool of the
Ubuntu live CD. (It includes gparted, which can do what you want.)