Here is the information from the Palacio de Bellas Artes web site. It's sort of hidden. The bottom line is that there are no online museum ticket purchases and they only take cash at the ticket window for the museum entrance. There is an ATM in the building. Museum Hours Tuesday to Sunday, from 10:00 a.m. to 6:00 p.m. box office hours Tuesday to Sunday, from 10:00 a.m. to 5:45 p.m. General Admission:  $80.00 MXN Photography permit: $30.00 MXN (applicable to devices other than cell phone cameras)  Cash payment only.  Admission includes admission to temporary exhibitions and the permanent collection of the Palacio de Bellas Artes Museum available on the day of ticket purchase.  We do not have reservations or online sales. The purchase of your ticket is the day of your visit at the Museum box office. Free entry: National and foreign public: teachers and students with valid credentials, older adults with credentials, people with disabilities and children under 13 years of age. On Sundays the entrance is free for national and foreign public.

No idea. Perhaps you could try their website or Mexico City Tourist Information

Es muy bueno y no requiere na formal, solo no shorts o playeras, camisa y pantalon falda o vestido esta bien

Go to the site web site and trust their answer

One and one half hour is enough time for this attraction. Special attention should be paid to the murals on the walls. An additional attraction in the area is the National Museum of Art (MUNAL). MUNAL is only a block away from the Palace of Fine Arts. You should spend at least three hours at MUNAL if you decide to go.