Sierra would be an appropriate OS to run Final Cut Pro 7 as long as your machine is compatible with it. Check Apple logo -> About this Mac. Which year and model is your computer?
Partitioning without an external drive to install macOS Sierra is possible. You just have to create two new partitions instead of one - one for the OS and another temporary one for the installer. Once Sierra is installed, the temporary one for the installer can be erased - and its capacity can be merged with the one that has macOS Sierra installed.
The partitioning and dual booting process can be a little bit involved, and isn’t really well documented online. So feel free to send me an email at treeandsunshine@icloud.com and I’ll be more than glad to walk through it with you via phone call or FaceTime.
As of fixing the project import issues in Premiere Pro CC, it again depends on the particular setup of your machine and projects - it would be easier to discuss over phone call or FaceTime. A possible solution is to first import the XML timeline into the standard version of Davinqi Resolve, which can be downloaded for free from the Mac App Store, and the re-export a XML timeline from Resolve for importing into Premiere Pro CC.