howdy arielle,
for the seedlings you best bet for faster growth is to put them in the garden in the ground this will produce better growth, wireing and pruning can still be done and the trees will recover better from damage. after a few years you can dig em out if you want.
as for the geode idea i would go for it.it's not the most "natural" look,but it would be cool
we'll all need better pics of the trees to help out with styling ideas