This is more of a Project Fi question.
The answer is - you CAN. You won't get the network switching that is a main selling point of Fi, you also won't get support from Fi if you have trouble with the service. Early on there were also reports of people having their account suspended if they used the main SIM in a non-Nexus device for an extended amount of time. I have not verified those reports, nor heard of any happening recently but those reports were on G+ that I have not visited in a few months.
I see you also posted in the Fi. forum. It's best not to cross-post to avoid parallel discussions.