A question about a recipe: Lentil sausage kale soup

I have never used red lentils but from what I understand, as the others have said, they cook faster and are better for a puree. I am guessing that the red lentils would give your soup a very thick consistency. I normally use regular brown lentils that you find in the grocery store. I have also used beluga lentils which held their shape. Both were good.

It would still taste great, but the texture and appearance of the soup would change. Red lentils cook down to a puree in the cooking time for this recipe. A possible way to use them where they stay intact is to leave them out while you cook the rest of the soup; pour boiling water over the red lentils so that they absorb the water but do not lose their shape; then add them to the soup for a final short simmer so they can pick up flavor. (Compare to making split pea soup.)
This is a technique I use for making tabbouli with bulgur and red lentils. You can see how I do it here -- http://www.food52.com/recipes/10549_red_quinoa_red_lentils_a_salad

Although lentils are generally interchangeable, I find the red ones cook much faster than either the brown or green lentils. So perhaps adapt the cooking time for the dish, or add the lentils later.

Personally, I wouldn't sub red lentils here.

This is not my recipe but I don't see why not. You can kick up a rucus on which lentils are "best" for anything (in Italy they will go with Casteluccio from Umbria) but largely they are interchangeable.