Sandals are absolutely fine. As a mark of respect for the Church, shoulders and knees must be covered however.
As long as your flash is off, photos are permitted in the Museums, EXCEPT in the Sistine Chapel, where photography is forbidden.” Flash does enormous damage to works of art and almost every museum in the world forbids it.
Students who are less than 26 years old need to bring their photo student identity card, they must also bring proof of their date of birth. Students in high school should also bring their student ID cards.”