Well the Ottomans themselves would be enough to take the Genoese possessions, there's little need for Venice and certainly none for Monaco, even if it was somewhat enlarged. The thing was that, like Venice, Genoa as a trading nation repeatedly used trading agreements to ensure control over its far-flung outposts. When Genoa's power declined around 1400, the Ottomans saw little reason to continue the agreements and cut off and captured all the outposts. Before that point, it saw enough value in Genoese trade to guarantee the Genoese sovereignty. Likewise the way that the Venetians kept control of their far-west Mediterranean holdings; they bartered for their safety, and occasionally the Ottomans got a little belligerent and attacked one place but respected the other deals for the other outposts. Outside of these agreements, a war only with Venice is the more like scenario, as the two trade republics tended to show contempt and hostility to each other for centuries.
Monaco, all in all, would be a liability. Any war with Genoa and Venice/Ottomans would be fought in the outposts, not in central Europe. By including Monaco, you're just making the chance of Monaco themselves being conquered increase, because Venice wouldn't bother with sending troops to help out. And if Genoa is threatened in its homeland, other countries will get annoyed and jump in - Savoy and particularly France often interfered with Genoa, and France repeatedly occupied Genoa as a "Genoa is our sphere of influence, hands off" type of message. Later on, they fell into the Spanish sphere, but by that time they owned no land east of Corsica anyway.