SuSE will let you resize existing partitions from the installer.
As for dual booting, one would hope that the SuSE installer would pick up the settings for that too. If it doesn't, the easiest thing is to install the SuSE bootloader to its root partition, keeping the redhat bootloader in the MBR, and then update the redhat bootloader to give an option to hand off to the SuSE bootloader.
When I say bootloader, I mean grub.
-Your RH install is in /dev/hda1. You have the redhat bootloader installed in /dev/hda
-Your new SuSE install is going into /dev/hda5. During the installation, you change the options for the bootloader installation so that it goes into /dev/hda5 rather than /dev/hda.
-Now you have two bootloaders, one in /dev/hda, and one in /dev/hda5. At this point, your system will boot to RH just as it did before.
-From RH, edit /boot/grub/menu.lst (or whatever the grub config file is called in RH), and add a new entry along the lines of
[Note that grub names drives a bit differently to linux: hda1 is (hd0,0), hdb4 is (hd1,3) ]
-Now, your redhat boot menu will include an option for SuSE: when you select this option, you will switch to the SuSE boot menu.