Oh boy. It seems to me that we've gotta go a little carefully here.
Let's start with your proposed layout. Draw your own schematic based on your current understanding. Maybe some questions from us will help you think it through.
How many zones do you have? (All thermostats will tie into the boiler's thermostat circuit to provide the call for heat.)
What load will the circulator circuit handle? You may be able to run both taco's off the boiler power if it's up around 4 amps or so. Otherwise you'll need to use the relay on the crculator circuit.
What boiler controls are in place and what are you adding? The schematic shows where the added controls should be in the circuit so all you need to do is identify the additions and the appropriate wires.
You'll have to supply power to the boiler and circulator relay (if used).
Draw your schematic with notes on wire colors and identify control or function. Reference your wires to the manufacture's schematic so function and location are clear to you. Write your questions right on the schematic.
Take your time and think it through based on function.