Use ext2 as the additional new partition, as it will permit to store the information more tightly.
Personnally, I would resize the usb key as much as you can as finally, you want to reformat it to its original state after. Formating a new partifion under linux is usually a lot faster than under windows. But if you are more at ease with
I would back up the critical data just in case. Sometimes there is loss of data while resizing.
Then as explained above use the porteus installer and target the partition that you have just created. Don't forget to select install bootloader. The new partition shoud be different from /dev/sdax, so rather /dev/sdbx or /dev/sdcx or /dev/sddx, where x is a number. If you are not sure provide as asked previously output of fdisk -l command.

Finally, just follow fanthom's suggestions. Persistence in graphic mode is already added when you are on usb.