It's important to give them room to grow. Over time (depending on the coral) they will grow to fill the empty space. Nothing happens quickly in this hobby.
You can find information on most corals on LiveAquaria and similar sites. Personally, I don't place corals too close together to start of...except zoas. I'll put them right next to each other if I feel like it....can't really hurt much. As things grow, they tend to figure things out for themselves I've learned. When the rare coral that has specific needs comes up, they will usually list that as an issue. Coral warfare can happen with certain things...if you start to notice something odd you can typically move one or both of the corals.
Placing them actually IN contact with each other is not a great idea, though. Like I said, they will grow to fill the gaps.
