Last time I was in Rome I brought a 10-22, 24-105 and 30 1.4. The 10-22 is a definite must for the colloseum, the forum, inside the vatican, and when you're right up in front of fountains in tight piazza's like trevi fountain. Your 50mm will definitely be great at for night shots of buildings, squares and inside museums where it's pretty dimly lit.
I definitely missed my 17-50 2.8 while I was there, especially the f/2.8, but one thing you want to think about is how much lens swapping you want to do. It can be a pain if you are constantly changing lenses in tourist locations between your 10-20 and 55-250. The zoom definitely comes in handy for closeups of the huge vaulted ceilings inside the Vatican, the statues outside at the top of the Vatican, and shots from the top of the dome.
So I'd probably recommend taking your 10-20, 55-250 (I wish they had that lens when I went!), and maybe picking up or renting a sigma 30mm 1.4 if possible. Grab a polarizer or a GND 6 hard filter if you plan on a lot of outdoor shots. Have fun!