The developer license you are using is designed for development and test purposes. It is not appropriate for production use. For production you will want to set the devices up for proper side-loading.
See Try It Out: Sideload Windows Store Apps on TechNet for how to enable side-loading on your systems.
If your Windows 8.1 Pro systems are on a domain then they don't need a separate key. You just need to enable them for side-loading. See Windows 8.1 Update: Sideloading Enhancements
The Windows RT systems can't join the domain so they'll need a key. If you're enrolled in a volume license program (see the Sideloading Enhancements link for which ones) you should be able to get side-loading keys from that. If not then you can buy an Enterprise Sideloading key through the Open License program.
See the Windows Volume Licensing Guide and Sideloading Licensing Changes FAQ