Is a magnesium complex better than a single form?
Not better, but it hedges. Useful if you do not yet know which form suits you; a single form is simpler and cheaper once you do.
Should I take magnesium powder or capsules?
Powder lets you start small and build up, which suits magnesium well. Capsules are simpler if consistency matters more.
What if magnesium upsets my stomach?
Take it with food and start at half the suggested amount for a week. That resolves it for most people.
Can I take it with a multivitamin?
Check what the multivitamin contains first – most include some magnesium and it is easy to double up without meaning to.