Yes, you can use a different marinate than the suggested powdered ranch mixture.  I do not suggest leaving the chicken plain because the marinate adds to the flavor of the kabobs.  My family also loves to do an Italian dressing marinate for the chicken pieces.  Just cut the pieces as normal and then add about ½ cup Kraft Zesty Italian dressing.  Stir to combine and marinate for 1 to 8 hours in the refrigerator.