Tim, for your fire, I would plan on cleaning out the firebox, and building a full size wood fire covering the grate. Add coal on top of this. This way you will have a fire over the whole area.
A coal fire will migrate sideways into unburnt coal, but only when the fire is burning hot. [in my experience] . I think if you try building small fires in one end or the center of the firebox, you will be frustrated with fires going out, and not burning right. And shaking good coal down in the ashpan, wasting it.
I'd clean it out, and wait till the weather gets cold enough for a good coal fire, [in a few weeks from the weather-guessers latest forcast] . If you want to make a small fire, wood does that well.
I'm shutting off my stoker this morning, 67* today, and rest of week, lows around 55* for the week.