6013 looks great on AC and it's easy to run makes decent strength welds. 6011 looks ugly but it burns trough rust and oil and make decent strength welds. I made these welds with 1/16" 6013 on DC. But you should have good results on AC. I would use a 3/32" 6013 Lincoln or Hobart.The 1/16" rods are very flimsy and hard to weld with. The 3/32" is stiff enough and wont blow trough 1/8" wall as easy a 6011.
I would use a 3/16" for all of it. It's not that much more expensive and it won't rust out as fast. It's also a lot stronger. 1/8" is strong enough but it will flex a lot more than 3/16". How far apart are you connecting legs? The part that go's from the handrail to the flat plate.