If I buy the X Pass through Google Play Store for my phone and tablet can I also use it on my Firestick or would that not work since it doesnt use Google Play? I dont want to have to pay twice.
First of all: SeriesGuide is not supported on the Fire TV stick. Only on the Fire tablets (and phone). Also note that on Fire devices signing into SeriesGuide Cloud will likely not work as it requires Google Play Services.
Anyhow: you can purchase on Amazon and send me proof of purchase (via email at uwe@seriesgui.de). Then I can gift you the Google Play version. (It does not work the other way as Amazon has no gift code support.)