Sometimes it is good to start with one and add as you see how it effects you.
However,you should know that B vitamins are produced (should be produced) also by the digestive track, especially the colon. If you are B vitamin deficient, it is much more effective for the long term to cleanse and repair your colon to produce its own instead of adding vitamins. Taking the B12 will be more of a band-aid than a real cure. I assume you are somewhat interested in cleansing if you are in this forum. Also, taking acidpholous may be more effective than taking B12.
For your candida, cleansing the colon and liver will go far to improving your condition, destroying the poor environment which caused it to get out of control to begin with. Also, you just can't take enough probiotics and they do real wonders.