I do not recommend using Project Fi on an unsupported device unless you fully understand the limitations and have above average tech skills to configure some features and troubleshoot.

A non-exhaustive list of limitations/issues using Fi on an iPhone:

1. iMessage/Facetime may not work. There are issues activating these services.
2. MMS may work, but you'll need to set the APN yourself, and there's some trial and error involved.
3. Group SMS/MMS will have issues, particularly if you're mixing iPhone and Android users. Best bet is to leave iMessage off if you're going to do this.
4. Obviously there will be no carrier switching between CDMA and GSM, so in the US, you will have T-Mobile service or none at all. Internationally it should be a little more steady since most carriers outside North America and parts of Asia are GSM.
5. Wifi calling probably doesn't work.
6. You won't have visual voicemail. You will likely need to dial out to check voicemail. Not sure about notifications.

I'm sure there's plenty more. If you're just looking for the international features and making some calls (not Facetime or Wifi calling), texting via SMS (not iMessage) and using data, it'll probably work. Just be ready to pop in a different SIM if it doesn't, since Google isn't going to help.

I'd say this: If you were an original iPhone user back when it first launched, when AT&T was exclusive, and you were able to get your phone working on T-Mobile acceptably, you probably have an idea of what a current iPhone on Project Fi will be like. If you're comfortable with that, give it a shot.