Welcome. I went to Venice in 2012 with a Canon 5DMkII and a 24-70 mm zoom. Other than the occasional vaporetto, the only means of getting around in Venice is your own feet, and let me tell you: that Canon weighed a ton after just an hour of walking around. This experience was part of the reason I returned to Leica M-cameras.
Venice is a complex warren of narrow walkways, footbridges, and stairs, and no matter where you want to go, there's never a simple straight line to get there! Turn right, go up some steps, turn left and go 100 feet, go down some steps, go through a a narrow alley, go up some more stairs...you get the idea. Trips that would be 1/4 mile as the crow flies are a mile or more in Venice. I don't describe this to discourage you...in fact, the absence of cars, bicycles and skateboards is part of Venice's charm. But wear good walking shoes, and travel light!
If you plan to see the great art in Venice, you are going to want a fast, wide lens. Paintings (Titian, Veronese, Tintoretto, Bellini, Tiepolo...!!!) are either displayed in museums like the Accademia or the Museo Correr, or in the many churches of Venice. Either way, the conservationists have made sure the light levels are relatively low, and tripods are usually prohibited. Also, in the churches the paintings are usually displayed as altarpieces or in some other religious/architectural context which you may want to include in the photographs, and moving back is not always possible. The 50mm Summicron is fine for the museums, but the 35mm Summicron or 28mm Summicron would be better for the churches.
The wider lenses would also serve you well for exterior architectural shots and "street shooting." The walkways are often too narrow to move back from a building to get a good shot of its design. And to be sure, Venice is rich with renaissance, mannerist, and baroque structures. Both wide shots and close-ups of details are everywhere. As for street shots, the narrow walkways tend to compress the crowds into tight groups, except where they periodically open onto campos, or plazas, where you'll find outdoor cafes, buskers, assorted performers, etc. Even the fondamentes (walkways adjoining the canals) are relatively narrow.
If I was faced with your choices, I would keep the 28mm Summicron on the camera and bring the 50mm in a comfortable shoulder bag or fanny pack or whatever. Use the lenshoods...flare comes off the water as well as the sky. If I had only one lens to bring, it would be either the 35mm or 28mm Summicron. I don't recommend the 21mm because its too slow for use in dimly lit churches and museums.
Other wonderfully photogenic places are the colonnaded shops and cafes along the Piazza San Marco, the Doge's Palace, the Scuola Grande di San Rocco (with it's staggering collection of masterpieces by Tintoretto), and the dramatic Basilica de Santa Maria della Salute (gorgeous day or night, near or far). The centuries-old Pecheria (fish market) is also a great photo location, especially when the catch come in, early in the morning.
I hope this is helpful. Have a wonderful trip, and travel safely!