Good thought process.
IS, is just basic car an upgraded version of Corolla.
The ES is slightly upgraded version of Toyota Camry.
GS is Sports Sedan.
SC, is a Sports Coupe
LS is top of the Line most expensive.
About the conotations. These are most US market. You seem to have a good idea, only an old Jap would know. Sedan might be correct conotation for S in the LS, ES, GS, IS, as in US the word Sedan is used in place of Saloon.