While I agree that you don't want all the trees to have too close of an appearance, too different doesn't work either. I see quite a few Larch forest made of collected trees that have one or more really out of scale with the majority and to make them mimic being scale they are chopped at the top. I find these artistically wanting. Nothing, is more unconvincing than a tree with no taper and a flat stump top in a forest. All my forests happen to be of collected saplings, mostly of similar age and size. In a pot, they will grow at different rates depending upon location in the pot just like they do in forest, except you can favor some to be taller, shorter, wider to the left or right, forward or rearwards, as you choose, and over time. Kevin's are too small right now to do anything with other than give as much space and good growing conditions as available to grow them individually to get to at least >1/4"/7mm, or bigger before assembling a forest. Of course, if he can stumble onto another few larger brothers in the meantime, great!
Assembling a forest of candidates too small will necessarily have trees too close to have enough headspace in years forward. Insufficient space for some individualism of canopies means just one united mass of a top and essentially no room for layers of branches open to the front, too. Wait until the trees get to the point of filling up some real space, or waste a lot of years, because they will grow very slow when put in a forest pot. I overpot my forests for beneficial growth, and repot up over time to continue that. And, they continue to grow. Growth leads to taper and the kind of mature appearance I want. It's been said, "In the end, we all get what we really want", meaning, we end up with a reflection of the amount of effort we have been willing to exert. Want a little, or want a lot...