Well, you are correct in just some cases. Caesium has only one stable isotope, so beside the actual Caesium and Caesium-133 everything else has ß-decay. Unless you are burning 100% refined Caesium, there is always a chance that you are also burning its radioactive isotopes.
All the isotopes of Cs other than 133 exist only in negligible amounts in nature. So sure, perhaps there would be a trace amount of something else-- but that is true of anything.
