Never used squaretrade but Preferrd care is excellent with the Pixel phones - If you go into the chat on the phone for support or call the support they instantly offer quick replacement with next day.
I know you can't change an existing pre-order, but his Google care something I can order separate without having to replace a new order?
Based on my experience from last year you cannot add it. I would give them a call to make sure because someone posted elsewhere that you could but it may have been through Project Fi. I'm of the opinion that it must be purchased at the same time still. I wish Google would change that policy.