[Note: These comments are based on my experiences with 700 and 900 stokers. I've never specifically dealt with undersize coal in a 520, so I'll defer to others who have more direct experience.]
1. I wouldn't be bashful about giving it air, especially if your coal is tending towards undersize. For any given feed rate, your most efficient operation is going to be achieved with the air setting that gives the proper ash ring around the top of the stoker. With undersize coal, it wouldn't be surprising if your air setting needed to be high relative to the feed rate.
2. I'm not sure what your definition is of a "perfect fire". As you move toward max output, the fire may tend to get kind of rowdy - a bit loud and unruly. If you get something reminiscent of a blacksmith forge, you're doing ok. In the bigger units, if the fire really gets enough air, it can be more like someone left the engine from a jet fighter in your boiler, and they forgot to turn off the afterburner. As one of these units works harder to deliver more btu's, the "dancing ladies" run for cover, and you're left with more of a primal blast of fire. To make steam efficiently in your situation, I think that's what you want.
Good luck and please keep us posted.