Q:

Arduino connecting multiple GPIO pins to one voltage line

So basically, I want to connect a small Peltier cell to an Arduino. I know that Peltiers are very inefficient and require large amounts of power in general but I only need to create a small temperature change. Anyways, I tried connecting it directly but the amount of current I get from one pin is too little.
I was wondering what would happen if I connected, say, 5 pins as parallel connections on a breadboard. Theoretically, if each pin is giving me 40mA, I should get an output of 200mA. Is this logic correct? Furthermore, if this is true, would I have to worry about reverse current spikes damaging the Arduino?

A:

Using multiple pins in parallel will share the current but there is a max allowed current per pin and for the entire mcu, you should keep the current within specs for both.  
The Atmel Mega 2560 datasheet, in section 31 (electrical characteristics), says:

but note that this is absolute max, so you shouldn't use that levels in normal operation. The normal operating current seems to be in the level of 20mA since that is the mentioned current in several parameters in the spec list and the graphs.
Also note that the pin voltage drops as the voltage you sink/source increases

I think you are better off using a driver for the Peltier cell, you can can use an IC, a mosfet or a transistor.

