If you want to use the coal boiler in the summer for domestic hot water heat, you can use its domestic coil. If you don't want to do that, you can use an L6000C strap-on aquastat, mounted to your coil's hot water outlet on the oil boiler, which would start up a Taco 007 and circulate the coal boiler with the oil boiler. Unless you have an on/off switch to the oil burner motor, it may start up when the cool water slug hits the oil boiler's aquastat probe. This won't accomplish your plan of having the oil boiler kick in during an outfire at the coal boiler. You should have a L6006C at the coal boiler, mounted to it's outlet pipe, so you don't keep feeding coal during an outfire.
You can accomplish almost anything you want with a reverse-acting aquastat. Also known as a B aquastat. The L6006 will solve all your problems.
As far as electric usage, the Taco 007 is a great circulator, and it's the only one you need. It uses .71 amp. Doing the math, if the circulator ran 24/7, you would use 2000 watts a day X your kw rate; about 20 cents a day for PPL customers.