Do the potatoes cause you any symptoms? Some people have trouble digesting starch fully or get a blood sugar spike from it. If they’re making you feel better without side-effects, then I think your own needs should outweigh candida dogma. People seem to forget that candida generally resides in the bowels. If you’re not digesting the starch or getting a blood sugar spike, then it’s seriously bad news for a yeast overgrowth though. Cooking the hell out of them and eating them with fat tends to help, but there might be better options.
I don’t think serotonin is quite as awesome as you think. It inhibits thyroid hormones and has other nasty side-effects. Improving the way it’s used, rather than increasing it is likely to help with your depression, since to do that you’ll need to improve your digestion and metabolism.