First you are going to have to explain how the "Republic of Sonora" and "South California" come into existence and when. I may be wrong, but I believe Mexico formally abolished slavery after Texas independence in 1836, so if the Republic of Sonora fissioned off from Mexico after that without slavery before its annexation by the USA, I think it's unlikely it would become a slave state. I have similar questions about "South California". Also, both Sonora and New Mexico are arid. Can SW states really support the intensive agriculture that makes slavery economically viable? Cuba makes sense poltically and culturally if you can make up a realistic scenario for its seizure by the US in the 1820-1850 period.
I think your best chance would be the following:
- Texas divides itself into several different states immediately after admission to the US (something I believe was legally possible under terms of annexation)
- The Indian Territory (Oklahoma) is formally dissolved as an Indian reservation in the 1840's and opened to white settlement. Since the "Five Civilized Tribes" moved into the territory were already slave-owners and most white settlers moving into the territory in this situation would come from Texas, Arkansas, and Missouri, it would almost naturally be a slave state.
-US purchase or annexation of Cuba, Hispanola, and other European colonies in the carribean where slavery was permitted historically. The problem with these options is that I doubt the USA in 1840 would admit states that are primarily spanish speaking with large mixed-race populations.
.