The Install Packages command does only that, push .pkg files. To install applications that have their own installers, you have a couple of options:
1) You can copy the installer to the workstation via the Copy Items command and then use either the Open Application/Open Items command or run a script to launch the isntaller. Note that most installers require user interaction, so you'd then have to take control of each workstation to complete the installation.
2) You can perhaps build your own package with the components of the application you wish to install and then push out that package. There are a number of package-building utilties, from Apple's own to JAMF's Composer.
In addition, some application installers actually contain a standard package within the installer "wrapper". You can try right-clicking on the installer and see if a "Show Package Contents" command appears and if so look in the container and see if there are standard .pkg files within. If so, you can probably push those out to your workstations.
Spotify is just a self-contained app, though, not an installer, and just needs to be copied to the workstation and then configured. You may be able to configure on your own system and copy the relevant preference files and avoid having to configure the app on each system.
Regards.