Redmine

1) is it possible to run RM using an external db
for authentication?
(e.g. RM users would be created "automatically" on
first login, like i have done with phpbb)

You would have to script it to tie Redmine into another system. You can use a Rake task or script/runner to run the script.

2) is it possible to easily (i.e. not hacking) turn off
the wiki and the forums and anything non-essential?
(we already have those, well populated. plus simplicity is good!)

Yes, just don't select those modules when you setup a project. You can also turn them off via the Project Settings panel.

3) how stable is the git support? is it considered production worthy?
(we're new to git, so don't want to be fighting multiple things at once)

Pretty stable. It's missing a few features (e.g. view multiple branch) but it works fine. I use it on all my git projects.

Eric