My suggestions for playing a Pathfinder game set in Carcosa.
Pathfinder classes available in Carcosa:
Dark Tapestry Mystery
Use the slow advancement track for experience. Severely limit magic items, in fact no standard type magic items should be in play. NPC's will not provide magical help for any reason, including healing or curse removal. Any character with skill ranks in knowledge Arcana, The Planes or Religion my attempt to use rituals. No prestige classes or multiclassing.
Many creatures in the Pathfinder Bestiary make excellent foes in Carcosa such as Serpentfolk, as well as pretty much anything reptilian, fungoid, slimy, or tentacled.