How do I determine which liquids are immiscible with one another?
If you want high accuracy for solubility, UNIFAC is a good way to go. The basic idea behind it is to break the molecules down into groups then generate correlations for how each of the groups interact. The weighted sum of all the group contributions gives you the total molecular interactions. From there you can get solubility.

The Dortmund Data Bank maintains a comprehensive list of all the group coefficients and offers a program to get the various properties, like solubility. Unfortunately, the free version is pretty limited. You an only access a handful of fluids. 

If you want to put in a bit more work, someone has put an Excel version on the internet. It doesn't explicitly do solubility, but basically if the activity coefficients are very far from 1, you can assume they are insoluble.

Furthermore, I'm not sure what the ferrofluids will do. I would guess that the concentrations would be low enough that you could just calculate solubility using the surfactant. Otherwise, you might have to search the literature for how to handle magnetic nanoparticles with UNIFAC.