Pay attention to the recipe cook time. In this instance, the recipe states to simmer the soup for 30 to 40 minutes, so you want to choose a rice that cooks in a similar amount of time. For instance, don't use a Minute or Instant rice that cooks in just 5 minutes as it would be way overcooked by the time the lentils are cooked through. Another tip is to rinse the uncooked rice under cold water until the water runs clear. This removes much of the starchiness from the rice so it won't clump up and get mushy.