I've yet to see a 500 with a fire fighting belly tank so I cannot help you there. Personally, I reckon that if you are under 1100 litres, it is of little benefit, and only begins to be really beneficial over the 2500 litre mark. IE everything less than a medium Bell (205/212/412) like B206, B407, AS350 (including B series but at a BIG stretch maybe not the B3), BK 117, A109, and H500 series, should avoid belly tanks as a waste of fuel burned for water dropped. Medium Bell range can show good use of belly tanks, but the larger machines are where they really shine (B214 and up to that great bit of kit on the erikson S64).
The bigger the machine, the harder it is to handle the bucket manually, the less of a factor the extra weight of the tank is in percentage terms, the more damage a dropped bucket would be in an urban environment, and the more likely the aircraft will be used in the critical urban fringe requiring overflight of built up areas.
So why would you put one on a H500? Or rather, what agency would actually request one?