There are many rules for hydronic heating piping, some good, some not so good. The rules were created for installers (plumbers) when there were also the designer of the heating system, sizing, equipment choice, installation, etc. Today all of these choices are much easier with the aid of computer design/selection software. Since each installation is unique, a optimum design for your installation can only occur if you do the design and select the design trade offs important to you. The single most comprehensive hydronic heating design software is offered by http://www.hydronicpros.com/
There are many others, many that are much better in their specific area. But, the free or trial versions offered at Hydronic Pros, are sufficient to design the typical residential hydronic heating system.
I too, I'm not sure of the question you are asking. While all boilers have many similarities, coal boilers are unique because they can and do make substantial heat when there is no demand. The expanding heated water has to go some where. If you don't have a dump zone you have to oversize your expansion tank. If it overheats and pops the safety valve, depending on the location of your circulator pump/check valve, it may not recover automatically when it cools and the automatic fill valve refills the system. Why? Because the pump impeller may not have any water in it, because a closed check valve prevented water flow to it. For this reason I put the circulator pump on my coal boiler on the return side.
My system is a primary/secondary oil and coal boiler system. Zoning is done with circulator pumps. I bought into the belief that pumps are more reliable than zone valves. Not sure that's the case today.