how is the tankless w/ a boiler system for heat

The tankless really isn't recommend for a boiler application. I'm not sure why. They want the loop to be "open" so that fresh cold water will supply the loop. Warranty is practically nothing on a closed loop that isn't drinkable.

If the hot water currently come from the boiler system you would need to separate the two and install the HWH in it's place. If you currently have a hot water heater there is no impact from a boiler system for heat.