Your video files do not have to be in your Photos App to load them as sources into Airmix.
You can take advantage of your iPad’s Files app to load them, whether the file is synced on iCloud Drive, Dropbox, or another cloud storage service.
1) Open the Apple Files app.
2) Navigate to the file you want to import and tap it.
3) Tap the Share button in the upper right corner.
4) In the list of apps, scroll over to find Airmix
5) Tap Airmix to load your file into the Airmix app
The video file will show up in the Source pane, ready to switch to.