I don't know about PF2, but I've used PF1 monsters in Starfinder without any problems. Barely any conversion needed, except to split AC into KAC and EAC (with EAC generally 1 or 2 points higher). There's a few other discrepancies, but it's more-than-likely the players will have no idea you're not using an "official" Starfinder foe.
EDIT: for accuracy.