if u don't have a pass, it could be very expensive when travelng by train from paris to roma.
for stay in roma, u can stay as long as u like, venice and Florence at least two days each.
for milan, if u don't stay for long, u don't see its charm. but i know that people are interested in this fashion city, so go there to see how long u like to stay.
if u can figure out the timing before ur trip, u can decide the hotels, otherwise, go there first without hotel reservation.
i am sure ur second trip to europe will be different.
come first, don't be afraid.