If not, is it too late to purchases skip the line tickets separately? it also seems strange that I did not have to select a day and time. I suppose I can use the tickets any time?
I bought tickets several years ago online beforehand and was able to skip the line - however - my tickets did have a specific day and time I believe. They only take small groups through with a guide if I remember correctly, so I believe one way or another you would need to get a specific time allocated.
When we were there at the height of the season in 2014, we arrived bright and early. I think a small line formed about 10- 15 minutes before the place opened up (9:00 at that time). Once the doors opened up we were inside in now time. Failing obtaining a timed entry, being there early can't hurt.
I recently bought tickets for the Secret Itineraries Tour at the Doges' Palace. Those are for a specific time.