I need to add the 'entity' content type field to the node content type, I'm just curious if adding this myself via an add-on could potentially cause issues if XenForo adds it at some point, or another add-on adds it
Each type entry is now associated with an add-on. That generally just means last one in wins. If we added an entry and then someone installed your add-on, it'd overwrite our add-on. Conversely, if they upgraded after installing your add-on, our entry would overwrite. So it's not really an explicit problem unless they ultimately aren't compatible.