Hello and welcome. I have no personal knowledge of Sycamore but they are used.
I am just mentioning in general, not about a sycamore in particular.
[quote]What age the tree should be when i start pruning leaves + roots[/quote] Don't do much in the way of pruning to young trees, it sets you back, early on you want growth. Root pruning though should be performed every time you re-pot or transplant. It is never too early to begin promoting a good nebari though. Have a look at the sticky thread about root pruning.
You gotta work up to that growth, it's not just something you just get in a bonsai. What I think, not saying it's accurate.
[quote]How to miniaturise leaves[/quote] This is only an issue after the trunk is established. If I understand your situation it is too early to be worrying about this. You have time to learn as the trunks grow out, research the terms 'defoliation and ramification' as they relate to bonsai.
The trunk might take more years to establish while those little leaves are multiplying, as long as it serves it's purpose as a trunk, even when it is little and thin that is the main thing. If once it gets to a stage and is the type of bonsai you can cut back, just because you'd like to, and it grows from root stock, and you cut it back you might have large leaves sprout, and it's possibly not something you can just start over on doing that, but one can try what one likes. Defoliation isn't something that I personally ever want to have to do after messing with roots long enough to get little bitty leaves.
[quote]How to avoid hidious stumps when i cut my sapling to be bonsai[/quote] The general idea is to allow the trunk to grow out then cut it back. A new leader is chosen from the resulting growth and it is allowed to grow freely thus healing the wound where it emerges. This is the general method of creating movement and taper with deciduous material. It is a long term process. Study these links and you will begin to get the idea.
The older trees that get too thick of a trunk and leaves are not enough to support the trunk stems and roots, that taper might be something, just kind of hacking into it a bit through the years working up to it making it kind of uniquely cut into and healed into, could prevent the hack and taper, if the leaves stay little, or if a big one pops out it might be of some helpful use. Personally it's the if, that I find to be entertaining. Lot's do not realize just how much the roots get messed with on different bonsai, on some of them, but the ones like juniper, you don't have to do much. I could be wrong..... Whatever is one's preference, and whatever one learns about the hobby as benefit. I am not here to tell you what to do, but it is apparently alright to mention my opinion. None of this is directed at the quoted or o.p. Have a nice day/night. http://www.evergreengardenworks.com/trunks.htmhttp://www.bonsai4me.com/AdvTech/ATdeve ... bonsai.htmhttp://www.bonsai4me.com/AdvTech/ATfieldgrowing.htm