Are you saying you have a 512 MB flash drive or 500 GB USB hard drive? Perhaps
Puppy could install to a 512 MB flash drive, but Ubuntu is too big for that. If
you meant it is 500 GB, then you can have lots of room for whatever you
decide...many distros are aware of each other and add themselves to the Grub
boot menu...more advanced users can point you in the right direction.