Also are there any simple way to disable the journaling completele on /rootfs or must that be done when partitioning it?
It has nothing to do with partitioning, but with creating the filesystem. You have (I think) an ext3 rootfs, you could convert it to ext2:
tune2fs -O ^has_journal /dev/sdx2
The partition should not be mounted, so you can't do it from the rootfs itself. You could also change your initramfs to explicitely mount the rootfs as ext2. BTW, it is also possible to put the journal on a different stick, though I never tried that.
After a bit more reading I tried to put /tmp and /var/log in RAM by editing "/etc/default/rcS" with..
but after reboot they still appear on the USB stick, any ideas what I'm missing?
Are you sure? These environment vars should force Debian to mount a tmpfs (ramdisk) on /var/run and /var/lock, but on a running system you won't mention it, if you don't look at the output of 'cat /proc/mounts'.