Power demand calculation
Current

Yes the right way to calculate current required is to work it out for each module and add them up. However, if you are combining Arduino modules, a practical way is to assemble them, power from a relatively high power source and measure the current. The only danger here is that you'll overload the Arduino's 5V or 3.3V regulators. I'd check that the specific Arduino has overcurrent or short-circuit protection.

Rather than measuring for all 5 daughterboards at once, you may be able to devise test programs for using them one at a time. Make measurements, then check the power supply path is adequate before combining all of them.

Voltage

The voltage regulators on a typical Arduino are not expected to supply much current. See Arduino and/or regulator data sheets. As Ignacio commented, it may be advisable to provide for an off-board regulated 5V supply.