Taco system control question

I am trying to figure out what I need for my heating system. I have one zone of baseboard, one zone of staple-up radiant (covers all but one room), and an indirect water heater. The boiler is a Biasi B10-3, and the total heat loss is less than 30,000 BTUs.

I don't know if it is overkill or not, but I am considering an injection mixing system for the radiant.

Looking at the Taco SR-503 expandable instructions, they show a setup with high temp, radiant, and domestic hot water, but they use two controls, plus the expansion mixing control. Their example has multiple zones of high temp radiation, and multiple zones of low temp, where my setup is much simpler. Do you think I can do it all off of one 503, or do they have a reason for using one for the high temp, and one for the low temp?

I know the mixing control has an outdoor sensor, a return sensor, and a boiler sensor, so it controls not only the speed of teh mixing circulator, but also the boiler temp, which wouldn't be used when the high temp side calls for heat.