Yes, an integrated smart thermostat or its remote sensors can be used instead of a Flair Puck to read temperature in rooms with Smart Vents, and also provide occupancy data. A remote sensor cannot be used to control a mini split, window AC or portable.
Keep in mind that every Flair system needs at least one Puck, configured as a Gateway and plugged into power using the supplied cable and adapter. This "Gateway Puck" is connected to your home WiFI and bridges all your Flair devices to the internet (even if you only have one Puck in your system).
Note 1: For large systems with more than 11 Flair devices, use the Large System Calculator to determine the minimum number of Gateway Pucks required for your home.
Note 2: Based on layout and construction, some homes will require more than one Gateway Puck to facilitate signal to other Flair devices.