Switching your cell number to Google Fi doesn't preclude using Google Voice. They're unrelated services, and you can transfer your existing cell number to Fi.
"Whichever option you choose, once you activate Google Fi service on your phone, you’ll no longer have Google Voice on the Google Account you use with Google Fi. "
It does, actually. When you sign up for Fi, you have to use your Google Voice number or lose it. The only way to keep both would be to transfer your normal # to a VoIP service that forwards to your Voice (now Fi) number.
https://support.google.com/fi/answer/6246240?hl=en
"Whichever option you choose, once you activate Google Fi service on your phone, you’ll no longer have Google Voice on the Google Account you use with Google Fi. "