Two options I can think of.
1. Use Select Where to select rows with values < 200. Make a subset using selected rows and choose Link to Original Table. Make subset of that using a random sample and also choose Link to Original Table. Now select all the rows in the final subset and they will also be selected in the original, because of the linking.
2. Make a new column with a formula such as :value < 200 & random uniform() < 0.2.