I have been using the Gotha 713 model for 15 years with pea coal only. I always bought coal from a local merchant. Since coal has risen in price, I started buying from sales on craig's list or other sites. Since some of these purchases included nut coal, I experimented with a mixture of both.
I always load my stove from 5 gallon buckets that are approximately 60% filled. This allows me to get a controlled slide into the hopper. I found that as I fill the buckets, If I mix in about 10% nut coal into the bucket, the burn is exactly the same as it ever was. The larger nut coal did not jam and I never saw unburned pieces in my stove after.
My conclusion is that pea is the way to go, but occasional use of nut is just fine.
Rich - Londonderry NH