Here's something I like to do when setting up the fish population in a new outing. If really big fish should be possible but fairly rare, I'll pick a couple of sites that have the best potential for monsters. There I'll set up 2 groups of fish: one with the main population, and one with a single fish that MIGHT be really big, or might not. Say, 10 brown trout from 8 to 14 inches, and 1 brown trout from 9 to 28 inches. There's only a 50% chance that there's a trout over 18 inches in that spot each time you visit it, and only a 5% chance of a 28 incher. I think that makes fishing such a stream more exciting and realistic, since you know the prime spots have a chance of holding something huge.
Paul