WaterHeater:Mixed and Boiler:Steam can not be on the same plant loop. Use and Source side nodes of WaterHeater:Mixed can only be connected to water loops. If user defined fluid type and associated fluid properties are specified in the plant loop, then also it won't let steam boiler to be on the same loop. The only work around can think of is leaving source side inlet and outlet nodes as blank and selecting Steam as heater fuel type. This will give steam end use in the summary.