This is two questions in one!
Unfortunately we cannot use the referral credit to purchase a Fizz phone.
Your referral bonuses are used up automatically as needed. Fizz use the total received from your referral bonuses towards your monthly payment. If your payment is less than the total received from your bonuses, you may then very well end up having nothing to pay for your plan. Any residual amount from your referral bonuses (the balance) will be applied to your next payment.
source - https://fizz.ca/en/faq/what-kind-of-rewards-do-i-earn-for-inviting-friends
The referral credit remains associated with the original plan that was accumulated on. If you have more than one plan and wish to associate your new referrals to another plan, you can go on your Fizz account settings to assign a different plan for your new referrals. To modify this setting, you can scroll down on this page - https://zone.fizz.ca/dce/customer-ui-prod/#/account/profile