Yes, B12 supplements work and they are often recommended to be taken together with folic acid supplements. As folic acid will cover up anemia, however, it cannot prevent neurological damage.
Yes, you can take them together. It is very rare that you will have an overdose of vitamin B as it is mostly excreted via urine.
No, they are not that effective however some vegans may prefer to take them
Yes, vitamin B complex supplements help in preventing neurological disorders and anemia.
Yes, they are more than sufficient. However, check your levels routinely.