I have 5 motors I need to control using Arduino. They are bidirectional (I've tested them). My friend suggested using a DPDT relay (such as this: http://www.radioshack.com/product/index.jsp?productId=13006053), which I don't completely understand in this context or how I can use it, but I from what I do know I think it's the best way. Since the Arduino outputs 5V, it needs to be 5V.
I have a prototyping board ready to be soldered on and I can get the parts. What should I do?
Tell us more.
What kind of motors ? Do you know the type, or can you make a photo of it.
If you tested them for reverse rotation, that doesn't mean they are also manufactured for that.
Do you know the voltage of the motors ? Can you measure the stall current ?
How do you want to control them ? With speed control in both directions ?
To control a motor with speed and direction, a H-bridge is used.
There are many shields for the Arduino with a H-bridge, but some of them are only for 1 or 2 motors.
If you only want full speed in both directions, the DPDT relay could be used. That could even be used with a PWM output of the Arduino to have also speed control.