thanks for the feedback.
I haven't been to Guarda, yet my experience with smaller towns in Portugal has almost always been satisfying. be aware that instead of the bus, you can as well take the train to Guarda (change at Coimbra). this said, without knowing more about your itinerary, it's difficult to tell whether stopping in Guarda could be a welcome break from a tour that includes mainly big cities or a waste of time in case you have more interesting places on your list...
alternatively, take an early train from Porto to Coimbra, enjoy, spend the night, take a train to Vilar Formoso the next day, carry on by bus. (you could switch in Guarda as well, yet since the train station is outside the city and you won't have time to look around anyway, it's likely not as convenient).
afaik, the busses have a toilet.
