Link Arduino Uno to Particle Photon I'm working on a prototype with a Particle Photon, but I've realized the required number of pins is bigger than the number of pins on my photon.

I thought I could have an Arduino Uno wired to some sensors, and link the Arduino to the photon via a Software Serial from the Arduino (as its serial pins will already be busy) to the Serial pins of the photon.

Is it possible, and is there a better way to solve my problem?
If you just need to get more inputs you could use multiplexing. The multiplexing method is more robust than software serial will be. How to do this is explained here and datasheet for the commonly used cd4051 here

You don't specify what kind of sensors you will use. if the sensors are i2c or similar it is possible to daisychain (multiplex too) them.