Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you are concerned that Molly ingested 200mcg of selenium.
In general, a single acute oral dose of selenium in the range of 1–10 mg/kg or 0.45- 4.54mg/lb (there are 2.2 pounds per kilogram) may be lethal in most animals.
200mcg = 0.2mgs (there are 1,000mcgs in 1 mg).
If she is a 5 pound cat she would have ingested 0.04mgs per pound of body weight. (0.2mg/5lbs = 0.04mgs/lb), not nearly a lethal dose, and in fact close to the normal daily recommended amount, therefore not likely to produce any signs of toxicity.
Signs of selenium toxicity include hair loss, lameness, anemia (low numbers of red blood cells), and liver inflammation, damage and scarring. But I don't believe that Molly ingested enough to be concerned about.
Please let me know if you have any further questions.