1. Different card can be used in different situation. Some store only accept Visa and some only accept MC. So in my opinion, 3 card is enough (one Visa, one MC, and one AMEX). Too many cards are dangerous. You won't be able to track your expense.
2. "line of credit" is the maximum money you can borrow from the bank. (correct me if I am worng)
3. Money transfer between AMEX Saving and other banks are free. No minimal amount required.
4. Other good places (I mean secure places) to put money is money market and bond. But that depend on your target (capital gain, security, or flexibility), there are lots of options.