Shoutbox

1 - You can add a gmail account as long as they have it activated for use through Windows Live Messenger.  It is possible to use any e-mail account through Messenger as long as it is verified and as long as it is verified then you will be able to chat with them.

2 - Yes, adding to the same chatlogs is possible as long as you copy them back over to the same place.

And make sure that your settings are set to use the same place.  If it's Messenger Plus! Live settings then you can use the Export Settings Wizard to do this, if it's Windows Live Messenger's own chat logs then you can just check the settings from inside Messenger itself

Though this only applies if you're doing a fresh install.  If you choose to "Upgrade" then nothing will change, you only need to worry about this if you are doing a 'Clean' install and formatting your hard drive