Hi,
Yes, Photon is cross platform.
You can adapt your game to work with Photon, but the most difficult part will be refactoring the feature to take in account the fact that a feature behave differently depending if it "owns" the GameObject or not. So I strongly suggest you really study the demo provided and until you haven't fully understand the implications behind implementing a character controller using Photon, don't start editing your game right away, I think you could loose some time and get frustrated especially if your game is almost done.
Don't hesitate to ask for clarifications and more in depth description of what is required and the design patterns.
bye,
Jean