I didn't know about this method, could have saved me much time, but the tricky part for me was to move /usr and /var loosely discussed by ruskie (I thank him a lot) and an example was given somewhere for n800 (not very usefull for n900). Now that I have moved these directories, I could install sdk (and every software i want) without worrying about rootfs or any partition. Finally, notice that my solution only requires a N900 and fingers.
[root@Nokia-N900 ~]# du -kx /usr | sort -n | tail -n 10 7604 /usr/lib/gstreamer-0.10 14164 /usr/share/microb-engine/chrome 14844 /usr/share/microb-engine 15632 /usr/share/locale 23352 /usr/lib/locale 24288 /usr/lib/microb-engine 25620 /usr/bin 67504 /usr/share 185396 /usr/lib 285068 /usr [root@Nokia-N900 ~]# du -k /var | sort -n | tail -n 10 120 /var/lib/hildon-application-manager 192 /var/lib/gconf/apps/osso/hildon-desktop/applets 212 /var/lib/gconf/apps/osso/hildon-desktop 380 /var/lib/gconf/apps/osso 572 /var/lib/gconf/apps 644 /var/lib/gconf 33808 /var/lib/dpkg/info 41040 /var/lib/dpkg 42064 /var/lib 42260 /var