So I copied an existing template quote layout and customized with our logos/colour theme/etc., but all new quotes default to "Quote (Galaxy w/Cover)" when I bring up the Deliver window.
How do I change the default (for all users) to use the new customized layout we've created?
Give the layout you want a right-click ;)
This is set per user.
You can change this for the users by going into Settings manager and changing the file name to the new file name of the layout you want them to use.
If you don't want them to use the other templates, I often create an 'Archive' folder within the layouts folder and copy the redundant templates in there - that will remove them from this list.
haha oops, the only thing I didn't try!
Your second suggestion was even better. I've removed the ones we won't be using so that just makes it a no-brainer.