Is it okay to take the form of a legacy race or creature from Pathfinder? Or is that illegal?
For example, could I cast polymorph as a 1st-level spell to take the form of a lava gnome with lowlight vision and fire resistance 2?
Edit: Okay, that was a bad example. I just realized lava gnomes have neither lowlight vision or fire resistance. XD
Alternate Example: ifrit with darkvision and fire resistance.
Edit: S*~@. Those aren't legacy anymore.
Well, you get what I mean.
Well your GM would have to port them to Starfinder and approve them. And once/if Paizo printed a SF version that would presumably override or change your existing version unless your GM said otherwise.
But theoretically those Pathfinder things still exist, they just don't have official rules in Starfinder for you to adopt.
Yeah, that sounds like the kind of thing where case by case GM discretion is the only good answer.
I would generally say, if there aren't published rules for it, that it's not generally acceptable.
However, it wouldn't be out of line to ask your GM for certain specific creatures that you could change into and what that might entail mechanically. Presumably most races of creatures from Golarion are represented among the Stars, even if in small numbers. They just don't have rules yet.