Hi mate, I've not heard of people having issues with coral being too close together. The only issues i thin you would have is if they tough each other at all (grow into each either etc) that and as things grow some of the middle corals may get reduced flow.
i have some SPS really close together an they're all fine :)
look at this SPS nano (would love to do this one day) it looks amazing and the SPS are VERY close together