What is the significant difference between pastas?
Pasta as you know is a simple egg and flour mixture.  You can add flavouring to it to give it a different spin or even a colour but at it's root it's just an egg dough.

The different type of shapes are "meant" for different types of sauces in a traditional sense.  I don't want to imply that you MUST use one type of pasta with one type of sauce.  What I'm saying is that a particular style of noodle shape just lends itself better to certain suace types.  I.E. penne, rigatoni, macaroni work really well with a sauce that needs a place to sit and hide so that each mouth full gives you the right amount of sauce to noodle ratio.  Mac &amp; Cheese is a great example of this.  

In a soup or salad the type of pasta you choose is probably going to reflect the need of the guest and how you plan to serve the dish.  

With a soup you want to have a spoon that will carry the right proportions of broth, vegetables, pasta in each bite.  Because of that you select a small pasta shape like orzo.  However, as cooking is all about using what you have on hand, if you don't have orzo but you have fetticini then use that but break it up first so it will work the same way.

Hope that helps.