Connecting arduino to existing pcb power grid
Assuming it is 5V, the main problem would be the current the device can provide,  beyond what it needs.  Your Arduino and wifi shield will likely take up to 400 mA by themselves at peak. If the device uses 700mA and it's internal regulator can only do 1A, then you would be over driving it. If it's directly powered by a external supply, you could replace the supply with a beefier one. 

But stop assuming and measure the voltage.