The goal of creating multiple income streams should be to maximize your potential in each category available to you.  If you are just starting out, it really isn’t reasonable to expect you to generate tons of rental income.  However, if you start maximizing your income generating potential through your primary salary, you will find yourself having excess income that you can reinvest to generate additional income streams and earn more money.
Portfolio income can come from multiple sources – interest/bond coupons, stock dividends, financial strategies including derivatives and capital growth.  Each offers some cash flow and some also offer potential capital gain with some risk from liquidity and volatile prices.  In this low-yield environment, many investors rely too heavily on cash flows and to pursue this are venturing further into riskier areas than they’d normally consider.