Giardia leaves a lot of dogs asymptomatic and sometimes the test comes back negative when it's present. It's treated when symptoms are present. It sounds like he will be fine.
I have given both drugs and all was fine (did have vomiting with the Panacur which occurs in 1 and 200 some odd cases). If pup pukes, clean it up and move on.
Don't give at the time time so if the Panacur is vomited up, the Metro isn't also vomited up.