How closely to space them is your call. Depends mainly on the size of the trees, size of the container, and the style or 'design' you're looking for. But don't make them all equal distances apart. Group some together more and a few farther apart. You want them random enough to be convincing, usually with a number 1 (featured tree) as a focal point towards the front of the group. A well placed stone or two can make the scene realistic.
Find some good pictures for examples. Using those that feature tree types similar to yours determine what styles appeal to you, or not, and use these things as a guide.
Yes, trees in groves or forests will grow together and pruning will shorten branch lengths which will push more fullness through back budding. But that is for later on. First off just let them grow and get established roots.