You will not have any problem to sleep when you arrive to towns. Albergues in Spain, any albergues in Portugal too, and usually you will have no problems to find a room in any hostal, it's not difficult in late september.
I just came back from walking part of the Camino Portugues this month. One dofference I saw is that, while there are many places to choose from when looking for accomodations, it's not as many as on the Camino Frances. My companions and I didn't stay in the albergues, though.
But there seem to be less traffic on the Camino Portugues than on the Frances, so you might not encounter that much competition for accomodations along the way.
I hope you are able to walk the Camino Portugues in September. It's a quieter route, and walking through vineyards is definitely a nice bonus