In the field, I think the most gonodactlylids I ever found in one breadbox size piece of coral rubble was 22. It is not uncommon to find 2-4 in even a small grapefruit size piece if rubble is scarce. These can be the same or different species. Often times they can coexist because there are different sized cavities and each animal just fits into its own. Bigger animals can't get in to evict a resident and smaller animals would not even try.
There is also another possibility. Many gonodactylids including N. wennerae pair for several days prior to the female laying her eggs. The male guards the female and keeps all other males away. As soon as she lays her eggs, he leaves. Sometimes in the the field if their is another cavity in the same rock, he simply moves around the corner. He recognizes his former mate by odor and will not attack her for a month until their eggs have hatched and the larvae have entered the plankton.
The "babies that you see, if they are swimming in the water column, could be larval stomatopods. If so, they will be gone in a couple of days. They can't survive in an aquarium for more than three or four days.