This may have been covered before, but if the PCs are on another plane (Carceri for example); the PCs are considered outsiders there, right? What about other creatures on that particular plane (slaadi, demodands, etc)? Do I have to figure out what the native plane is for every creature encountered or are they all native to that plane since they are not on the material plane? Any help please.
No, the PCs are not considered outsiders; they gain the "extraplanar" subtype. There's a big difference.
This may have been covered before, but if the PCs are on another plane (Carceri for example); the PCs are considered outsiders there, right? What about other creatures on that particular plane (slaadi, demodands, etc)? Do I have to figure out what the native plane is for every creature encountered or are they all native to that plane since they are not on the material plane? Any help please.
They are not considered outsiders - outsiders are physical souls, where a humanoid is a soul with a body. They are, however, considered to have the Extraplanar subtype, as is every creature that is not native to that plane. Normally extraplanar outsiders on their home plane instead have the Native subtype.
Though Slaad are normally native to the plane of Limbo, as DM you can consider them to be native for Carceri (or any other plane that uses the Abyssal themed encounters), and just assume that enough pure Chaos exists on the plane to tie a visiting Slaadi to it (or to spawn new slaad). I'm pretty sure that Demodands are native to Carceri or Gehenna, but you can easily assume the same thing as the Slaad but with Carceri being a font of Evil as well as Chaos.
TK