I was in venice in 2006. I took my Mamiya 645 on a carbon fiber monopod. It was so much easier than a tripod.
When I was there, I saw one 4x5 on a tripod and one or 2 35mm camera on tripods. It also depends on the time of year.
If you go when its slow, i dont think anyone will really mind.
IIRC there is a tourist help line that could confirm if there are any new regulations.