Indeed, the salts are probably better to work with, unless there's a specific reason not to.  That being said: cells in general tolerate 10% DMSO - for freezing.  They probably wouldn't do very well for very long with that much in culture media.

In general, 2% DMSO is the recommended maximum - in assays of recombinant protein.  Cells could probably tolerate it, but for how long is questionable.

I generally use very small concentrations of DMSO in cell culture (less than 0.2% in general), and have found that to have functionally no effect on any cancer cell I've thrown it at - during a 6 day proliferation assay.

That's part of the problem - how much DMSO cells can tolerate depends upon how long you want them to do so, and what sort of system they're in.  The easy way to answer your own question is just to run your assay looking at 5-10 different DMSO concentrations, say starting at 2% DMSO and working downward from there.  The data will tell you how much you can use to help solubilize your treatment.

The fatty acids should be dissolved in 100% DMSO. This concentrated stock solution should then be added to the medium, bearing in mind the tolerance of the cells to DMSO, as already mentioned.

Palmitic acid is soluble up to 20 mg/ml in DMSO. Oleic acid is soluble up to 100 mg/ml in DMSO.