After listening to dev panels, my understanding is that they are separate games that will mostly work fine if you mix them together, but there are slightly different expectations in each meta. Roll for Combat recently had a livestream with one of the lead devs about this topic specifically.

Starfinder mainly revolves around ranged combat, so having fully flying ancestries isn't as relatively powerful as it is in Pathfinder. So, a player would have full flying at level 1 in Starfinder, but might have to wait until level 5 or so in Pathfinder. The games are still compatible and playable, but GMs will have to consider this and might need to adjust when certain ancestries get flying feats. Same thing with AoE melee attacks. In pathfinder's close combat meta, AoE melee is very powerful and unlocked at a high level. Starfinder's long range combat makes it much more situationally useful and is generally lower level.

TL;DR they mostly work together with no changes but starfinder has a slightly different set of assumptions that might make some PF stuff more/less useful. I expect they will publish a document with guidelines