Some (if not all - not too sure) urchins eat Coraline, thus you may be waiting a long time for it to develop.   
 
Do you have the lights for SPS?  I would not suggest it as a beginner coral, difficult to keep particularly with summer heat coming.  Personally I would add only one chromis now (as they can get territorial as they get bigger), and the hammer.  The best of both worlds - just watch your water quality, which shouldn't be a problem as it is still a low bioload. 
		
	
		
		
		
		
			
		
		
		
		
	
		
		
	
	
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