Part of working with city states is making sure that an individual city can be viable in the game. You can get some ideas of what I do by looking at my Naval Mod when it comes to increasing terrain yields and boosting resources, along with increasing the effects of irrigation and mining. The greatest effect of my changes comes with coastal cities, as there are a number of ways to increase the yield of coastal, sea, and ocean tiles. You can find them in the Naval Mod by going through the various city improvement.
For this specific scenario, I first had to convert Venice into a civilization that could be played by a human player. Now, Venice is not an individual city, but does have some other cities under its control during this period. Then, I needed to determine how to add to Venice the ability to generate colonists to settle open areas to increase its capability compared to the Ottoman Empire, its principal opponent at this time, although France, Spain, the Papal States, and the Holy Roman Empire all played into the mix. Venice was the leading trading power at the time, and maintained the largest Mediterranean navy behind the Ottomans. France generally was an ally of the Ottomans at this time, as Charles the Fifth controlled not only the Spanish holdings in Europe, but was also the Holy Roman Emperor, controlling a large partion of Germany and Austria. Fortunately, the alliance of France with the Ottomans is not reflected in the game.
To make Venice competititve, I gave it several improvement available only to the colonizing nations of England, Holland, Spain, France, and Portugal. Then after looking the units over, I had to add a gold mine to Venetian territory so that they could obtain Swiss Pilemen, one of the better infantry units. I also added some additional resources to help with increasing the population of the cities.
One thing in this scenario that helps enormously is requiring only One Food per city inhabitant rather than two. This makes from growing cities faster, and also makes increasing the maximum city population a much more reasonable thing. I generally go with 12 for a Town, 21 to 25 for a City, with Metropolis starting from there. I got that idea from TEturkhon's Test of Time scenario. If you are looking as single city0state civilizations, I would highly recommend increasing the Town and City populations under General Settings, and also probably changing the food requirement. That makes them much more vialble and also productive, and makes for a more interesting and fun game. Remember though, there changes to have a strong tendency to bite you back.
