I believe with ginkgo trees, leaf reduction is most commonly achieved through regular pruning in spring and autumn.
There are a few things to note however:
- the leaves do not reduce by much, so they are naturally more suited to medium to large bonsai.
- ginkgo trees don't heal well after pruning, so never cut flush to the trunk or concave. Leave a stub and remove it after roughly a year once it has dried out.
- when pruning back leaves, cut back to 2-3 leaves in the leaf cluster, making sure that the top most leaf is on the outside of the branch.
To answer you second question, the recommendation for starting to train a ginkgo bonsai is by using a 2-3 year old sapling. So if you are happy with the progress of the trunk and nebari, then I see no reason why you can't start with your 2 year old tree.
Good care instructions can be found here and here. Good luck with it!