I don't know about the addiction part of things, since the only ingredient that would suggest it is caffeine, which is not even considered addictive by the general definition.
As for radioactivity, normal Nuka-Cola seems to be irradiated by external sources rather than by adding a radioactive ingredient. There are several isotopes that can become irradiated after being exposed to even small amounts of radiation and the effect becomes stronger with heavier exposure. A prime example are vehicles used in the clearing of the Cherbobyl reactor, certain metals of which became irradiated and thus continue being radioactive 30 years after the initial exposure.
In the case of Fallout beverages it's safe to say that Sarsaparilla not being slightly irradiated is an oversight considering the rest of pre-war stuff giving you a few rads upon consumption.
Also, considering the fact that the Mojave was spared from being nuked directly, all pre-war stuff should give you way less rads than in Fallout 3, which was annihilated way more thoroughly.
Anyway, I think that the irradiation mechanic could be greatly improved upon by splitting the radiation types you receive and giving them different effects. While even strong alpha sources are rather harmless from the outside if you wear anything over your skin, ingesting them is really bad since they cause heavy damage to tissue and store all of their ionization energy inside your cells. Gammas, on the other hand, don't really care what you're wearing unless it's lead plating, but cause way less damage by depositing only a small amount of ionization energy in your body.
P.S. Some Stalker mods have a mechanic where food dragged through radioactive zones becomes irradiated itself, which could be a really interesting mechanic for a hardcore mode. You'd have to either eat the irradiated stuff, not carry any food with you or keep it in a heavy lead container on prolonged trips.
