I believe the rule you are reffering to actually reads the weir of the trap can not be above the vent pipe opening. The reason is to prevent siphonage of the trap. This doesn't apply to water closets as the trap is above the vent and actually relies to a certain degree on the siphoning action to suck water from the trap. The ballcock then ensures a trap seal. The answer to your other questions depends largely on the code that has jurisdiction in your area. According to the UPC all fixtures are for the most part individually.