Also, if I use different DVRs at different locations / sites, can I still use the DVR viewer software to view all the sites?
The CMS and EMS software provided with the iDVR-E4 allows users to view multiple DVR locations at the same time. Please note that this software will only work with the iDVR-E4, iDVR-E8, iDVR-E16 and iDVR-RT16 model DVRs. You can learn more about viewing CCTV cameras at multiple locations using the iDVR here:
Can the iPhone app also view all the different sites in one app?
The iPhone app will allow you to view different sites, however you will have to view the sites individually. The iPhone app does not allow you to connect to more than 1 site simultaneously.