GM Core
Chapter 2: Building Games
Scope of Changes
Source GM Core pg. 63
It's up to you to determine how much of your game you want to customize. Many GMs use the default rules and creatures and set their adventures in the Pact Worlds or another published game world. Other GMs devise and incorporate allnew creatures and places with strange themes that don't fit in the standard Starfinder game or galaxy. Neither of these approaches is inherently better than the others. The most important thing remains creating a story collaboratively with the rest of your group while having fun.
Determining what your group wants out of the game and setting makes a big difference here. If you're playing Starfinder with a major goal of exploring the Pact Worlds, it's more likely you'll use “stock” elements rather than creating new ones. On the other hand, it could be more interesting for your players to see brand-new things if they play with you to experience your own creative voice, or if they're experienced Starfinder players looking for variety.
Determining what your group wants out of the game and setting makes a big difference here. If you're playing Starfinder with a major goal of exploring the Pact Worlds, it's more likely you'll use “stock” elements rather than creating new ones. On the other hand, it could be more interesting for your players to see brand-new things if they play with you to experience your own creative voice, or if they're experienced Starfinder players looking for variety.