Crew Level
Most starship combat encounters are between characters of roughly equal level and skill, regardless of the tier of the starships they are aboard. If there is a large level discrepancy between the combatants, adjust the difficulty of the encounter up or down a tier to compensate. In any case, combats between crews that are more than 4 levels apart should be avoided.
In general, the level and skill ranks of an NPC crew member are equal to the CR of the NPC or the tier of the enemy starship (minimum 1). To determine the skill modifiers of an NPC crew member, first decide whether the NPC has mastered the skill or is simply good at the skill. Usually, one crew member will be a master at one skill; the rest of the crew will have good skills. The skill modifier for a master skill is equal to 9 + 1-1/2 × the NPC’s ranks in the skill. The skill modifier for a good skill is equal to 4 + 1-1/2 × the NPC’s ranks in the skill. Alternatively, you can determine NPC skill ranks and modifiers using the master and good skill modifiers for the combatant array from the monster building rules in the Starfinder Alien Archive. Of course, if you have full stat blocks for the NPC crew members, you should use their actual skill ranks and modifiers.
Source
Core Rulebook pg. 326