Flat feet shouldn't cause any probs in MA.
In TJQ we are also told that we should see the toes during stance training.
Knees should never got further than your toes and should always be aligned with your foot.
As for shoes, bassically any shoe should do, but afterwards go with the shoes to a good in-sole specialist and let him create in-soles for you.
Should take about 60~90 minutes for the consultation and maybe a few hours to get the in-soles done.
Don't by any ready-made, but get them made for you.
I would recommend atleast 2 sets for you:
1.) Sports shoes for MA.
2.) Shoes that you wear at work, especially if you spend a lot of time on your feet.
3.) Any other shoe that you wear on a regular basis.
Cheers.
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