In Polish Sausage Lentil soup, can I use some barley to replace some lentils - can I mix the two? Same question only opposite - in Turkey, Tomato, Barley soup, can I also use some lentils?
Yes to both questions. Lentils and barley can go together quite well in a soup. Here's a recipe for a lentil and barley soup as an example. Compare it to your soup recipe to see if there are any major differences (especially in cooking time or liquid to barley/lentil ratios). My hunch is that the barley will absorb more liquid than the lentils, so you may need to increase the water a bit. Just keep an eye on the soup as it simmers, and add more water as needed.
There's one caveat about your turkey, tomato, barley and lentil soup idea. Legumes take longer to cook when the cooking liquid is acidic. Tomatoes are acidic. Lentils cook quickly enough that they will get fully cooked even in an acidic broth, but it takes longer. (If you were using a slower-cooking legume, such as beans and chickpeas, I would recommend pre-cooking the beans.) Here's a recipe for a tomato, lentil and barley soup as an example. Notice that it simmers for an hour to get the lentils fully cooked, whereas the soup without tomatoes only has to simmer for 40 minutes. If you want the faster cook time, simmer the soup for about half an hour before adding the tomatoes. That will give the lentils time to get nearly done before increasing the acidity of the broth.