Lemons aka Lelupi get to be 5" stick with the Multi's and some Julidochromis Transcriptus they stay smaller rock pile on one end and a shell field on the other end instead of Multi's you can go with a small colony of Brevis..
word of warning, if you still plan to keep 2 "lemons" (im assuming your reffering to leleupi) than you should know that they have been known to kill off multies, so you may reconsider having them in the tank
I would only pick one shell dweller for a 30 gallon, maybe 2 but not 3. Also I don't think there is enough room for 2 shell piles and a rock pile in a 30 gallon. These guys need room to make territory.