How much power (amps) through 5.5mmx2.1mm power jack? Through a DC switch?
It is very unlikely that whatever DC "wall wart" jack you have can handle the current required by the coolers.  The 200 mA or so for everything else should be fine.  However, switches to handle 8 A are readily available.

You could directly connect the coolers to the battery because in theory the coolers shouldn't come on until controlled to do so.  A solidly off MOSFET should have little enough leakage to not matter.  I'd feel better with a master shutoff switch though.

Either way, you should put pulldown resistors on the FET gates.  You want to make sure the FETs are off when the processor outputs are high impedance.  10 k&Omega; to 100 k&Omega; is good enough and won't get in the way of the processor driving the FET gates.  The pulldowns only need to be low enough to overcome leakage and provide some impedance against external noise pickup.