No additional subscription is required to use it on iPhone - a single subscription covers up to 5 devices (any combination of iPads, iPhones and iPod Touches). Please keep in mind that iPhone/iPod version does not support charts, but you can still use it for ADS-B weather.
Regarding WingX question, no subscription for SkyRadar application is required if a pilot intends to use SkyRadar receivers with WingX (only active WingX subscription).