Yes, it is recommended to turn your boiler off whilst the tank is being filled and to leave it off for a short period afterwards of up to 30 minutes. This will ensure that any sediment that has settled on the base of your tank is not stirred up and drawn into your fuel line.
Many situations require one or more back-up boilers, especially those involving residential accommodation. A properly designed and installed biomass boiler will be as reliable as a fossil fuelled boiler, and in this respect the usual rules for specifying back-up boilers should be applied. However, as a biomass boiler is rarely sized to meet the peak load, the back-up boiler usually acts as a peak-topping boiler as well. Hence, it is usual for back-up boilers to be sized to meet the peak load.