This took a bit of searching.
Rede Expressos offer a service via Celorico Beira.
The timetable is to long to print here.
However, if you shoot over to my blog here:
http://barlaventostravelblog.blogspot.com/2008/01/bus-from-pocinho-to-guarda.html
You will find a png image of the relevant page from the timetable. Note I am not responsible for any changes to or inaccuracies with the timetable, and that certain services only operate on certain days.
The company Rodonorte offers a service via Viseu as follows:
http://www.rodonorte.pt/ver_horarios.php
Pocinho - Viseu
Depart: 08:30 Arrive: 10:45
Monday to Friday Only
Depart: 16:15 Arrive: 18:30
Viseu - Guarda
Depart: 10:30 Arrive: 11:20
Monday to Friday Only
Depart: 19:12 Arrive: 20:02
Monday to Friday Only
Depart: 19:15 Arrive: 20:05
Monday to Friday Only
Depart: 22:02 Arrive: 22:52
Sundays Only
Depart: 22:05 Arrive: 22:55
Note that many of these services only operate on school/weekdays.
Based on your requirement to be in Guarda at 20:42
to get the train the Rodonorte service via Viseu seems the best option on weekdays.
Depart: 16:15 Arrive: 18:30
Change Viseu
Depart: 19:12 Arrive: 20:02
Monday to Friday Only
Note however that Rede Expressos operate to Celorico da Beira where the San Sebastian train stops at 20:15. This may offer a better connection.
