[susan53]

1. Re your lion and rabbit - no problems with "become" as friendships do go on developing. You could limit though by saying soon rather than since then. The verbs are a bit weird, however, especially without clarifying time adverbials, especially to establish the reference point - ie the time of the action. I'd rewrite it as :

It was summertime, and the rabbit was thinking of his friend the lion. They had met at a Christmas party six years ago, and had become good friends since then.

orIt was summertime, and the rabbit was thinking of his friend the lion. They had met at a Christmas party six years ago, and had soon become good friends.

or, with this last one only :

It was summertime, and the rabbit was thinking of his friend the lion. They had met at a Christmas party six years ago, and soon became good friends.

The choice of the form of become depends on whether the writer chooses to see the reference point as still being summertime, as it was for the party (ie the events happen before that point and the perfect is used) or as the Christmas party (ie the event happened after that point and the simple past is used).