My lads use 6013's all the way, pipe at 6G, 2.5 root, same for cap, or a 3.2. Up to the welder to decide his preference.
I find root pene issues are down to, too long an arc length, keep rod within V prep, a little push of rod into pool helps to ensure root pene, don't worry if you lose sight of keyhole, the push, will help, you can also hear if you are burning through, kinda hollow noise to weld, hard to describe.
Only time I revert to Cells, if I have a complete numpty, which is rare!
i can get full penetration without burning through it just looks **** , think ill get some more fleet welds practice with them a bit more> i had a little go with 6013 today just added a bigger drag angle and moved faster , seemed okay , couldnt really see inside to check though