Yup, mangroves you have to give them room to grow out of the water. When you first get them they are usually small and are completely submerged, but they will grow quite tall. I would say Cheato is a better thing to have for nutrient exporting, but there is no reason you can't have both cheato and a couple mangroves in your refugium as long as you have the room vertically and can hang the light up high enough. How much depth do you think cheato needs? My refugium is only about 4" deep after to take into account all the live rock rubble in it, and the rest is stuffed to the rim with cheato.