coupla thoughts as you begin the work of establishing a forrest. Once new growth hardens off you may want to try and get some smaller diameter trunks started by clipping and rooting some new trees. I find to create depth of field in forrests you need atleast three different sizes of trunks. While this new collection is young and vigorous you should remove the largest leaves to try and create more of a uniform look to the leaf size on all the trees.
Keep in mind there is a reason why folks use privit as hedge material. It uniformly fills open spots with filler growth. So you will have to be diligent in keeping ( as John naka suggests) open spots for a bird to fly through....
In the japanese mindset you'll want to have an uneven number of trees in your final forest planting.recently I had a class in forrests where students had two groupings of trees in an oval pot. one of 3 and the other of five.....(perfect) until one student questioned and asked doesn't 3 and 5 equal eight? LOL
Over the years of working with forests it pays to have individual replacement trees in the three diameters readily available incase a tree dies and needs to be replaced. But nothing wrong with keeping a dead tree in the grouping to add more realism to the struggle of life in the forrest. Kinda up to your own discretion.
I have see some artists use small figures of forrest animals (bunnies, deer) that add a touch. Again up to your dicretion. Best of luck