How to mix pasta and sauce evenly?
Don't put the shrimp etc in the sauce in the first place and cook it separately, that way you don't have to take it out. Then just top your pasta with it and if say you are cooking meatballs and want a little sauce on them mix a little sauce in with your meatballs and then put them on top. If you want that mixed in with the pasta look then do the same thing I said above and mix the pasta in your individual serving bowls or plates if you are using one after you have dressed your pasta. 

Your just not going to get an even distribution with random variables, all you can do is pick and count if you want everything evens.