You can use any of the extras, extras-testing and extras-devel releases.
You can even use any combination of the three but you should only do that if you know what you're doing.

It's the same concept like mixing different Debian releases. I guess even pinning should work, but I never tried.
However, be warned: There are no clean upgrade pathes between Maemo releases like in Debian. This is why you'll often read posts that say "never use dist-upgrade".
And sometimes even the dependencies within one release are broken.


Maemo is Debian's evil brother in terms of repositories. You're at the gates of dependency hell!

So to summarize:
Always think before you act!

If none of what I just wrote makes sense to you (because you're not moderately familiar with Debian), then stick to extras alone, never mix releases and if you need a package from extras-testing or extras-devel only activate this repo to install the package, pay very close attention what's going to happen to your system and deactivate it right away.