Hi @Bárbara Fredericksen
Yes, you can use the Interrail Pass for this journey. If you can proof your residence in Poland with an official document, you can choose Poland as your country of residence. This means, you would have 2 journeys allowed within Poland. For more info, click here.
Regarding seat reservations: In Spain, you would have to make the seat reservations direct at the train station in Spain. There is an interesting Post on how to make reservations in Spain, but keep in mind that most of these options are not 100% confirmed.
For the rest, it depends on the route and the time you are planning to spend travelling. If you let me know the date, time and destinations, I can check it for you.
Have a lovely day!