Coral growth definitely depends on a lot of factors. Generally, there are some faster growing corals such as zoas, birdsnest, and some slow growing corals like chalices etc.
Spacing isn't too big an issue as each coral will make sure to keep the space it wants etc.
For some corals, I'm not a fan of plugs and remove them and attach to the rock directly. For corals which I want/need to keep the plug, I always cut off the stems from the plugs and just use the round discs and attach with some superglue. when starting out, I'm a fan of using superglue as you can remove the frag easily and rearrange if needed. With epoxy, once you've set it, you can't remove it without using a saw etc.