Yes, you can have all InDesign versions installed on the same computers. I have CS3, 5, 5.5, 6, CC, CC2014, and CC2015 installed and all work fine.
Yes, but do it with the CC license only, Otherwise you would waste your license and you will have a version with reduced capacities.
If you have CS6 from a permanent version installed, deactivate it first and uninstall it. From within the CC application you can install CS6 and all CC versions. CS6 from the cloud has more functions than from the permanent version and is able, with internet connection active, to open newer files and convert them.
Thanks both for the quick answers, its installing now, I only have a CC licence so more straight forward for me. Hope you both have great days