I agree that you should wait before adding any more SPS corals.Your tank is immature and needs more time to stablize. Now shrooms on the other hand should thrive in your tank. They seem to like trace additions and some indirect feeding(same for the xenia). In my tank I have shrooms that expand with iodine additions and the xenia seems to thicken and pulse more often. All of the xenia I have was from the same original piece and some pulse, some do not. Some are doing great in heavy current, while the others are doing great in moderate or slack current. IMO their reaction all depends on where they were collected or even how they were treated during shipment or at the LFS.
As far as food goes xenia and shrooms take in nutrients in the water that are present from a tanks regular feedings. Xenia seems to absorb nitrate from the water and help keep nitrate levels low as long as they are pruned occasionaly. Now you may find it hard to prune shrooms once they are established. Some even refer to them as "roaches".
Some say that xenia will flourish at a higher PH and ALK, and it is probably true in most cases. But some here even say that at a PH of 8.4-8.6 and a high ALK their xenia doesn't pulse or even flourish. But it does seem that xenia, in most cases, does fare better at a high PH/ALK. That along with some nutrient IMPORT(feeding) will help xenia flourish. A lack of pulsing does not mean your xenia is doomed. I have some xenia that never pulses, yet it divides and spreads well. Now shriviling or the loss of their heads is a different story. That is a sign of a problem. So by making sure your PH/ALK is up and there is adequate feeding, as well as keeping your salinity and lighting constant, may help your xenia stay healthy.
Good luck,