If the brakes are 12x2 in. they will be OK. Not sure where you got the 5 amp draw on the magnets. Most magnets draw about 3.5 amps per when fully applied. You should never need full application as you will lock them up first.
A 30 amp circuit braker shoud be fine. However I would suggest you up size the wireing in the truck to #10 as the factory wiring is not sized for 6 brakes and you will see a voltage drop over the undersized wire.
Now all that said I am currently drawing 21 amps with 6 brakes and have No Brakes and not sure what the problem is just yet. But then again when something is broken all bets are off.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles