As far as switched load the typical 24 VAC system is unlikely to ever see more than an amp or two. The largest transformer I have seen in a residential 24 VAC HVAC system was 100 VA and ran two furnaces and two central air units simultaneously.
I think it was still about double what was needed though. So I would think a 5 amp capacity relay on a 24 VAC system is more than enough.
As far as number of zones thats going to depend entirely on what application the unit is going to be used in. I doubt most systems would ever need more than three. Primary cheap fuel source, Secondary more expensive backup fuel source, and third most expensive last resort backup source.
One fatal flaw is that if the main single controller quits then all sources quit. For that reason when I set up multi fuel heating systems I use an independent thermostat for each system.
They are just programed with the proper time and temperature changes off set from each other. Secondary is set at 5 -10 degrees F lower than primary with the warm up modes starting around 20 -30 minutes later than the primary systems temperature changes if there is a temperature overlap between them.
The third system is the last resort fail safe usually with a old fashioned mechanical thermostat set for its lowest temperature possible. Usually around 45 F.
You may also want to consider that some commercial and industrial applications do use 24 VDC power as well.