If the itinerary that you gave in your original post is all you want to do, I doubt whether buying a EUrail pass is a good idea. You're likely cheaper off if you just buy regular tickets. The biglietto chilometrico is also a good idea, but I haven't seen it in recent trenitalia ads, so I'm not sure if it still exists.
As to accommodation, don't get your hopes up about couchsurfing: especially in Rome and Milan, it will prove hard to find a host as these are popular destinations in Italy. I'm reasonably sure that Milan has a municipal campsite that is easy to reach. Rome's more of a problem: either settle for paying more for accommodation, or accept that you will sleep far from the city center (which need not be a problem, as certain parts of town have very good public transport connections even at night).
update
The biglietto chilometrico ceased to exist ages ago. The Milan campsite can only be reached by public transport until 0:20AM. The journey to it involves three changes of metro/bus. If I were you, I'd find myself a hostel in Milan as well.
[ Edit: Edited on 06-May-2009, at 13:50 by bentivogli ]