RTT's about right, the most common gases by far are going to be CO2 or Nitrogen, and maybe some sulphur compounds simply because those are what gets erupted from volcanoes and is heavy enough to stay around the planet over a long time. In colder atmospheres (i.e. the outer zone) you're more likely to find methane, ammonia, and nitrogen. Helium is likely to be retained by very massive worlds (size A+) in the habitable or middle zones.
Corrosive atmospheres are likely to be corrosive because of acids (sulphuric most likely, then hydrochloric and then nitric) more than chlorine and fluorine - the latter are highly reactive and would have to be actively produced, and the only way to really do that is to have some kind of lifeform breaking down a chloride or fluoride and emitting the gas into the atmosphere. They are however very rare elements in terms of geochemistry, and so you're not likely to find them in large amounts (we're talking parts per million, maybe up to a tenth of a percent by mass in the atmosphere at most).
Insidious atmospheres are... well, a bit of a cop-out as defined. Venus has traditionally been defined as a "Corrosive" atmosphere in Traveller, but that's really not doing it justic - it's Insidious all the way. Between the high pressure carbon dioxide atmosphere (90 atms pressure at the surface), 400-500°C temperature hot enough to melt lead, and the sulphuric acid in the air, it's going to beat any kind of protection in pretty short order. Which goes to illustrate that "Insidious" isn't really about being corrosive, it's about all-round environmental hostility. Personally I'd class any world with high radiation or non-stop volcanoes (like Jupiter's moon Io) as an Insidious environment too.
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