Here is the jist, personally I go at the trees, any bonsai, straight from seed, as soon as that tap is long enough and those cotyledons are stable with a first and/or second set of real leaves. Really with those cotyledons thick full of genetic instructions for the plant, and those have to stay on or the plant dies, those roots might be too, so I figure after toying with the roots either I get little bitty second third and fourth sets of leaves or I don't. Occasionally I might remove a little cotyledon, and get better results. I have gotten leaves millimeters in length, and stay that way, to sprout and the plant lives, however what kind I am not going to mention, you can have your own unique experience without influence from me. Then again, for things like the better known bonsai material there are plenty of instructions, and I have found a lot of instructions to be poor guidance and large leaves result. I am fixing to start an American sycamore just for kicks, and see what happens, and it might take 5 different seed starts to come to a conclusion. It takes a lot of time to bonsai a plant and as soon as that root system is sound, it might not stay a little leaved bonsai and that might not change, once established, because you have to have a certain amount of roots for a plant to live. Personally I have found if you let the trees, bonsai material, that doesn't have published info about, get too large and grow to much, you have wasted time getting little bitty leaves and flowers, because you won't obtain it. It's a unique hobby, and just because I or someone else might mention what a result or a particular result doesn't mean you shouldn't study up on the plant your trying. A lot of info on how to maintain large trees and feed them comes in real handy when it comes to bonsai, it's about the same but the measurements are different and root pruning is a bit different. A lot can be done with root pruning, I'd say more so than plucking at leaves. You'll get a shape, raffia and wire or not. Nature, it does a lot for itself, and plucking and raffia wire and an inclination might keep your results minimum, for size it's one thing to trim. Just enjoy your time, and if you don't come up with results the first 15 or 20 etc times, there is always next time, and it's not a bad idea to have a larger tree on hand for rootstock should you need it. American sycamore, for instance, might require a lot of maintenance to achieve a result, not something one can just let sit for 6 months and forget about, but I could be wrong. I do not know yet, but I will soon enough.