Substitutes for Italian sausage

(Note: This answer assumes your recipe is calling for the product generally called "Italian sausage" in the US, i.e., this stuff.)

The soup should be fine using ground chicken or turkey instead of Italian sausage. The only thing to keep in mind would be that both chicken and turkey are leaner than pork (the traditional filling for Italian sausage), so you'll end up with a slightly less rich soup.

The dominant flavoring in Italian sausage is fennel seed. To substitute for a pound of sausage, I'd probably add about 2 teaspoons of fennel seed, crushed up in a mortar and pestle with some salt (no need to grind it all the way to powder, but crunching it up will distribute the flavor better than whole seeds).

Some Italian sausage also includes garlic and/or hot pepper flakes (the latter being the difference between "hot" and "mild/sweet" Italian sausage). Adding a clove of garlic or a little bit of chili sounds good to me, but the soup will probably turn out fine without as well.