Those areas might be closer to 20-22 ga, 16 ga especially is pretty thick.
My go to is well sharpened 3/32" electrode with 1/16" ER70S filler rod for steel from 0.035-0.095" thick. For really thin stuff (20+ ga) sometimes I straighten 0.023"/0.035" ER70S from a MIG spool. Some of my friends use 1/16" electrode and claim it offers a little better control. I've never been a fan of it since the electrode is pretty weak and tends to break (not just the tip) if you accidentally dip and have it extended to reach into a corner.
In terms of filler sizing, you really want filler small enough that you don't need to heat it directly to melt, versus melting it with the base metal pool if that makes sense. This means the filler should generally have a diameter below the base metal thickness. If you experiment with a few different filler sizes you'll see that sometimes you might need to run the torch over the filler to heat it in addition to just worrying about the base metal. My goal is always to minimize overall heat introduction into the work (above what's required for complete penetration), so adding extra heat just to melt the filler really isn't good.
Have fun and post up some pictures!
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Don't mean to sound like I copied Bob; he types too fast! haha