Sec. 250.104(A)(2) says that "in buildings of multiple occupancy where metal water piping systems(s) installed in or attached to a building or structure for the individual occupancies is metalically isolated from all other occupancies by the use of nonmetallic water piping, the metal metal water piping system(s) for each occupancy shall be permitted to be bonded to the equipment grounding terminal of the panelboard or switchboard enclosure (other than service equipment) supplying that occupancy. I would assume that you have bonded the building's service to its metallic water service piping, as required by the NEC. Water is conductive, but whether it can conduct enough ground fault current to trip a circuit breaker is a good question. Your question about the toaster falling into a metal sink while water is running is moot, since it would probably be plugged into a GFCI receptacle, which trips on much smaller current.

The "Rule of Six" that you're referring to applies to services and focuses on the number of service disconnecting means permitted for each service, per Sec. 230.2, or for each set of service entrance conductors, as permitted by Sec. 230.40. Per Sec. 225.33, it also applies to the number of switches or circuit breakers in any subpanels in situations where you have "more than one building or structure on the same property and under single management" being fed from the load side of service disconnecting means. In other words, yes, you are limited to six overcurrent devices.

I am not familiar with any electrical codes that apply in Malawi, si I can't comment on their requirements. In the U.S., we use the National Electrical Code (NEC). Sec. 250.24(B) of the NEC requires that for a grounded system, you must have an unspliced bonding jumper connected between equipment grounding conductor(s) and the service-disconnect enclosure to the grounded (neutral) conductor within the enclosure for each service disconnect.