I have never heard that you cant keep plants in the same room as a SW
tank. I dont know if their are basing their concepts on higher humidity, or if the extra lighting from the reefwould affect the plants some how or what.
As for topoffs to replaced evaporation then yes you top off with freshwater always. The only time you add saltwater to the tank is if you physically remove water from the system. And you dont want to ever add salt directly to the system but instead you want to premix the water and salt before adding it.