Fully Compatible
Source GM Core pg. 241
The rules for Starfinder and Pathfinder are fully compatible. Both include sci-fi and fantasy themes, and many thematic elements fit seamlessly within another. If your campaign tells a story of time travel, an alien ship loaded with futuristic tech crashing on a primeval world, or you simply want more options for your game, you might decide to allow PCs to choose options from both games.
Even though the rules are compatible, some options from Starfinder aren't a great fit for a Pathfinder game, and the reverse is true as well. For example, it's much easier to get darkvision and flight in Starfinder than in Pathfinder. If you find this interferes with your campaign, you could restrict access to any equipment, feats, or spells from Starfinder that grant darkvision, or limit ancestries and equipment that grant flight at 1st level.
Even though the rules are compatible, some options from Starfinder aren't a great fit for a Pathfinder game, and the reverse is true as well. For example, it's much easier to get darkvision and flight in Starfinder than in Pathfinder. If you find this interferes with your campaign, you could restrict access to any equipment, feats, or spells from Starfinder that grant darkvision, or limit ancestries and equipment that grant flight at 1st level.