What a mistake to be made!
I need to convince you that you are about to miss a fantastic, true Camino experience and the beauty of Porto. 30 minutes and 3 euro and you are right in the heart of Porto. Nothing more simple than taking the airport tram line E. 13 stops, you get out on Carolina Michaelis station and you walk 300m to Rua Barrao de Forrester 954 and you get to the Albergue de Peregrinos do Porto. Great albergue. It is your first Camino and you would have a chance to get the spirit from the very beginning. Late afternoon? Plenty of time to make great memories and meet your fellow pilgrims. And in the morning you just start walking from the albergue. It is located on the Camino. 2km of a very nice walk through the Porto centre (with quick stop at Sao Bento station - marvelous tiles) and you get to the Cathedral where you can celebrate a kind of symbolic beginning. You can then continue along the beautiful river Douro with great panoramic views of Porto or take a tram to Matosinhos.
What you want to do instead is to spent that time in an industrial area, doing nothing special and to begin your camino trying to figure out how to get to Matosinhos through the industrial suburbs.
Honestly please don't! My heart is bleeding imagining such a waste.
You can find that albergue on fb as well to ask if there are beds available. You can get your credential there or in the Cathedral.
I hope I managed to change your plans