However, I have a SIM card from before the announcement that I'd like to activate and still requires use of a Google phone to do so. I've read that I can do this on a phone that's not already activated on Fi, but that it will lock the phone to the phone # associated with the SIM. My question is: how do I dissociate the phone used to activate the SIM card from the number afterwards? Will this automatically happen once the SIM card is placed in a new phone?
Whatever you read is wrong. Google Fi doesn't play any games like that - are you thinking of Republic Wireless or one of those other carriers?
My wife & I have both used Nexus 5Xes on Fi, and when hers died, I swapped mine over to her with no problems at all (I had mine as a backup device while we figured out local internet reliability).
The downside of using a non-supported phone is that you're stuck with T-Mobile's network (I think). One of the real values of Fi is that it will roam between networks for us - we've got, between personal and work phones, AT&T (my phone), Verizon (a work phone), T-Mobile and Sprint (my wife's Fi device). We're pretty well covered, connectivity-wise, plus a pair of rural wireless connections. We don't use that much data, but it's nice to have the diversity when we're traveling.