As I stated above you need to add the sdk repository, and after an apt-get update, apt-cache will find it. And then you can install the packages. But make a backup first. When you have installed many application to your N900 it could be that you fill up your rootfs (because the sdk repository is not optified for the device partition structure) and it could be that the phone doesn't boot anymore.

But my studies are not over and you are on a cssu-thumb system which was made with gcc linaro-4.7 and I don't know how it harmonize. Best would be to have a second device on stock and install gcc-4.2 and make your first try to study and learning. After that you could make further experiments.
For thumb I would use the scratchbox environment.

Here to make it more clear what you need to add. The tools repository is maybe not needed.

Here are the used cflags in scratchbox for gcc-4.2:
-march=armv7a -mcpu=cortex-a8 -mfpu=vfp -mfloat-abi=softfp -fno-omit-frame-pointer -fno-optimize-sibling-calls

For the linker:
-rpath-link /usr/local/lib -L/usr/local/lib -rpath-link /lib -L/lib -rpath-link /usr/lib -L/usr/lib

deb options:
DEB_BUILD_OPTIONS=maemo-launcher,thumb,vfp

Cross your fingers, good luck!