This year the teams from Castille León have been included in Group 1 (alongside teams from Galicia, Asturias, Cantabria and for reasons I don't undertand... Murcia), group 1 would make it feel more geographical accurate albeit more challenging to get promotion spots.
As for rivalries I would go this way:
Major rivalries
Guijuelo
Valladolid
Numancia
Guijuelo is pretty much Salamanca's biggest team after UD Salamanca's disappearance so games against them could be considered as "Salamanca's derby".
Valladolid and Numancia are Castille and León biggest and most notorious team (number 1 and 2 respectively), and thus pretty much our targets to beat.
Minor Rivalries
Mirandés
Cultural Leonesa
Mirandés is a team that has started to gain notoriety in the last few years (specially after a spectacular run in the Copa del Rey where they made it all the way to semfiinals) and Cultural Leonesa while not going through its best moment it's historically speaking Castille and León's thrid greatest team.
About Zamora and Burgos, I'll leave it up to you. They're from the same autonomous comunity, but they aren't great nor famous compared to the others.
.P.S.: By the way, accept my apologies for making you dizzy with so many posts.