i think that you can install gtk2 normally along side 1.2.x i have both installed on my arch system. They are kept apart by how the directories are labelled. well at least i have not found any troubles with having both on my system.
as far as i know i don't think the 1.2 apps can use 2.0
yeah you can, but its a bit different. Goto gnome2's page and take alook at the garnome tool. It installs gnome2, gtk2 and all alongside of gtk1 and gnome1.xx. and, it compiles everything for you. So if you have 5 or 6 hours to burn, you can build it yourself.