1) The /tmp /home and other directories are on separate partitions, each having a certain amount of space allocated to use. The / or /tmp just refers to a mount point and is meant to keep some sort of logical hierarchy. So even though your root partition is a separate partition in its own right, you have a /tmp partition that logically resides under the root partition, but in reality is an entirely separate storage area. Files that get put into /tmp aren't being stored in the root partition, just like files that are stored in the root directory wouldn't be stored in /tmp (unless explicitly put there).
2) You can use a command line utility such as fdisk, cfdisk, or parted, or if you want a graphical utility you could try out gparted, qtparted, or kparted.
3) I'd try using parted/gparted/kparted/qtparted. If you've ever used partition magic, they're quite similar to that program. As to whether or not you'll lose data, you shouldn't, but as always when changing hard drive disc configurations, make a backup of anything that you can't afford to lose.
Hope that helps!