Why would it be dangerous to take vitamins? The Bs are water soluable so any excess would be flushed out. Taking just two of the Bs isn't going to help as they all work together. I take large doses of vit B12, a B complex, vit D3 and vit C. I should probably be taking vits E and A, too, but haven't got around to it.
Don't be afraid of going up to 75, it's a baby dose. I was on 200 at one time, plus a bit of T3. It's against your doctors wishes because he knows nothing about thyroid. Very few of them do. That's why we have to take our health into our own hands and learn as much as we possibly can about our disease.
The dizziness and weakness are more than likely due to low B12 because when we are hypo, we have low stomach acid, therefore difficulty absorbing nutrients. Dizziness and weakness are one of the many, many symptoms of B12 deficiency.
Muscle cramps, on the other hand, are likely due to magnesium deficiency - you're probably deficient in many things. Most of us are. You really should get some tests done - B12, (all the B vits perhaps), vit D, magnesium, iron, zinc, as many vits and mins as you can. They are so important and have such an impact on our health. And they are relatively easy to correct. Don't suffer, get tested.
Oh, and the ginseng, which one are you taking? Because one is better than the other. Also, it doesn't agree with everyone. Are you sure it agrees with you?