Niacin is well tolerated by most, if you can handle the flush.

These need clarification:
B complex
Mega potency Multi

There is B Complex which usually has lots of things and low amounts of most Bs and then there is B-50 Complex that has all of the Bs. Folate or folic acid is an important part, 500 mcgs or more. B-1, Thiamine is also good.
The Mega Potency Multi may be great or it may be of little value. The importance of a multi is getting the trace elements.