Displayed in order of oldest to newest:
This one probably dates from the 1860s/early 1870s based on the plain style of the card and the condition of the albumen print(s). No label on front or back, but a hand-written note in Italian stating “Palazzo Cavalli”
This one is still probably 19th century, but 1880s-1890s. American made.
Most likely between 1900-1910. This one fascinated me because I’ve actually been there! It was neat to see how the Piazza looked over 100 years ago.
This one came along for the ride in the batch of stereoviews. I’m not sure if this Venus resides in the Bargello or the Accademia.
dcphotoartist 
The last two views, of Giorgio Sommer, were produced between 1867 and ca. 1871. The photos were taken ca. 1865. I have researched Giorgio Sommer and Edmond Behles extensively.