I was using everytime HAM for adding repositories because it is easy to disable them and it was also meant by the design to use the gui.

Only the cssu installer added some repositories which I not added manually.

There are also links which automatic added some repos, but never used them better to control it and do it yourself.

maemo extras repo is compatible to stock.
CSSU repos are updates to stock which are system packages and I think thats why it is a different repo also not all packages could be made autobuilder friendly or simply the autobuilder was outdated.

Everything should be compatible with each other, only when you on cssu you should not install old packages from extras when new ones are in the cssu repository. And when you are not on cssu you should install it first before you are using any cssu repository.

This gpg error is because the public key is expired, so what can be done from our site is something like the following link or to change the date?:
https://askubuntu.com/questions/7434...-checks-of-apt

Nothing else to say Maemish was giving a good overview.