For the purposes of osmolarity, any old salt will do, it's just the electrolytes that cells won't let through that matter, i.e. sodium and chloride.
For reducing nitrite toxicity, it's the chloride you're after. I'm not sayin' that if your tank isn't cycled you can just add a ton of salt and make it ok, just that if you're in a fishy cycle, a bit of salt couldn't hurt.
Iodized table salt contains NaCl and NaI. Typically table salt is around 99.8% pure NaCl. A small amount of NaI (iodide) is added to prevent goiters for inlanders. You'll find that sea salt has more iodide than table salt.
I don't know what harmful chemicals QCppg is referring to, but I assure you, table salt is perfectly safe and completely appropriate for FW purposes, such as cycling and treating Ich.
However, if you want a brackish or rift lake system, then you're going to have to buy the appropriate salt.