A 29g would be a huge improvement on a 6g!!! But, as it already has been mentioned, they will in time outgrow this as well. Not tomorrow or next month but in a few years or sooner. A full grown oranda & moor can both max out over 12inches with proper diet & care. Please ask if you have any questions!
Respectfully, I've never heard of Orandas or Moors getting to over 12 inches. To my understanding, that's common, comet, and shubunkin territory.
In my opinion (and they'll vary), you will be ok with 29 gallons, but it is the absolute minimum, and you'll want filtration rated for a 50-60 gallon tank. Based on my reading, typical body lengths for Orandas and Moors are more on the order of about six inches (again, this is body length and does not include finnage), though well cared-for, 8 inches or so isn't unheard of. While specimens may have been known to grow larger than this, I would think it relatively rare, and on the tail-end of the bell curve, size wise.
Even at 29 gallons with double filtration, though I think it will be manageable, the tank will require frequent gravel vacuuming and water changes. You will also want to try for a tank with as much water surface area as possible to maximize oxygenation. That said, a tank larger than 30 gallons will make maintenance easier, so I think you should consider it if you can, but again, I think 29 gallons, in most instances with normal sized fancy specimens is manageable.
Best!
