Yes, there is no problems keeping them in a tank of that size providing lighting is of sufficient level, more so for the Rose...
Providing you have 1 - 1.5lbs of live rock per gallon of tank water, you wont be needing any other filtration. Ro water is good, but would suggest you only really need to change about 10% per week on the water changes.
Additive wise, if you using a good salt like Reef Crystals, you should not have to add additives as the salt itself should hopefully buffer your pH to somewhere around 8.1 - 8.4 providing your Ro water is around 7.1 - 7.3..The only thing that may need to be added is calcium, but that all depends on the levels of coral stocking in the tank. People to tend to have a need to add more calcium, or use a calcium reactor when mainly stocking hard corals, as softies dont really take that much from the water column..
Hope that helps
Niko