1) To get MSN you can use aMSN or telepathy-haze. You don't need both. aMSN is a seperate client, telepathy-haze integrates with the built in client. aMSN will eventually also allow video chat (it's being worked on right now). Both are in beta stages I believe.

Extras-testing is not necessarily harmful but it's not recommended that any user that's not familar with the n900's file system and troubleshooting. For example, files may install to the /root partition instead of /opt meaning your device will fill up with memory quickly and then you will keep getting out of memory issues since /root is on a tiny tiny partition.

Though if you really want too, you could install the telepathy-haze and telepathy-butterfly and then disable the repo. But again as I pointed out above, not all programs in extras-testing will necessarily install to /opt thus you will have to clean up the files yourself once those programs do go into extras.