It is hard to get too much B-12. For one thing, it is hard to absorb it. For another, it is water soluble, so if you get more than you need, it will wash out in your urine. Final thing, from deficiency to saturation takes much higher than normal doses for a time.
I'd recommend a sub-lingual type, since it absorbs better under the tongue than in the stomach/intestines. You do want to take it along side a regular b-complex. The B's work togther, so if you're missing one, the rest don't work as well.
Candida squeezes out the good bacteria in your guts, those that produce small amounts of Vitamin K and B-12. It is difficult in today's world to be healthy enough to produce enough of these vitamins, so supplementation helps, especially if you don't eat animal products (another source of B-12) or lots of dark green leafy veggies (another source of Vit. K).