the oncidium alliance plants might be better in the med and smaller bark, with the perlite and charcoal, unless the "small" bark is extremely fine, then adding the chunkier bark will help prevent the mix from being too heavy.
the 50/50 bark combo, perlite, charcoal sounds good for the zygo (some people don't recommend using different sizes, tho I have orchids potted in combo of different sized bark - cyms, paphs, some onc alliance - and haven't had any problems)
the den - I might go with just the bigger bark, perlite, and charcoal, tho it could depend on what type of den it is, how large, etc ...
I usually try to fit in some material for drainage in the bottom of pots, and frequently use styro pnuts - it's not necessary, but a number of people do this. I have also used, either with, or instead of styro - pumice, pieces of broken clay pot, the 'marbles' sold for fish bowls/tanks and flower vases.
If you use packing pnuts - make sure they are not the biodegradable kind - those turn to mush when they become wet, and defeat the purpose. Also, white or green is ok, but pink ones have been treated with something to prevent static, and shouldn't be used in plants
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