Glad you got it! Even more glad you have the chance to play with coal. There IS a learning curve. I hope you get to see that it's a good thing before the 400# runs out.
Too much heat? Isn't that a good thing? Heeheeee!
OK, seriously..... Got a CO detector? if not, stop using the coal! That said, you should be able to control it and the overheat would be an occasional thing. Usually over heat it from too much incoming air. Coal demands good air control. If your door gaskets are leaky or if the doors simply don't close tight, there's most of the problem. The next part is that coal demands accurate draft. It's pretty much mandatory that you use a draft gauge (manometer) to set the barometric damper. A Dwyer #5 is perfect. NOT a #7. Google it & you can buy one from the factory cheap as anywhere. You might have to phone them for #5. Next, coal stoves & boilers like a particular size coal. I'm going to guess you'd run better with nut size. Oops...sorry, I thought you got stove size. Perhaps your boiler would like pea size. But yes, you are thinking correctly to perhaps cover the stove size coal with pea size, that will slow down the air.
Comparing to wood, the only downside, if it's a downside, is that coal makes way more ash. For me, not much of a downside. If you boiler is where you have to carry ash up stairs, down the hall, through the loving room, through the kitchen, out through the entryway, through the garage, around the house and 1,000 feet to a dump place....well... then it might be a downside.
Welcome to the Forum and best of luck to you. Stop back anytime.