Udikas: How does the included mouthpiece play for you so far? If don't yet have a teacher, get one, then be guided by his/her advice.
In my experience, the mouthpieces I play on bigger saxophones have little to do with what I prefer on soprano, save for the fact that I like open tips.
Contrary to Tom's advice, I find Runyon soprano pieces to be distractingly small (both in bite and in sound). I'd imagine the included piece would play just as good as anything else you may seek out, at this early stage in your soprano career. Proper reed selection is probably more critical that the mouthpiece at this stage. DAVE