How will the current move?
This setup should not pose any problems so long as the servo tester, ESC, and BEC are made for the voltage you are giving it. 

With most devices, you just have to worry about supplying the correct voltage because the device has an internal resistance which will cause it to draw a specified amount of current. This is okay because the manufacturers designed it that way.

A good example is with a lightbulb. If you directly connect 110V from an electrical outlet, it will only draw a very small amount of current. It has the capability of drawing thousands of amps, but it only draws couple hundred milliamps. This is because there is an internal resistance in the bulb, such as many other devices out there, that cause it to draw only so much.