Chris, I would use 1/8" 6010 with a 3/32" gap and a nickel land. Be sure to "feather" your tacks which means thin it out with a grinder where the tack starts and stops ,but leave some meat in the middle of the tack. Most pipe tests I have taken allow use of a grinder. Usually, you can tack the coupon on the bench and then fix it to the stanchion where it is put in the 6-G. Examine the tacked up coupon carefully and should you have 1/4 of the coupon tighter than the other 3 quarters......make sure that is on the bottom (between 3 o'clock and 9 0'clock) because you want to do this quarter first. Assuming all goes well with the root pass, grind it out so it is smooth with no slag showing and then run your hot pass. I would use 90 amps and go as fast as possible(travel speed) and as far as possible with 3/32" 7018 and grind the stop & start. IF you travel too slow or use too much heat, you risk blowing a hole in a perfectly good root which could be a disaster. The key is really not to grind out too much and really pay attention to the puddle when you are laying down the hot pass.
Once you have a good root & hot pass, you can crank up the heat even more which will help burn out any slag, plus will help eliminate porosity which is the major cause of failure of pipe tests, IMO.
Good luck with that.
3 c's: CALM, CLEAN, and COMFORTABLE. And don't be afraid to try a bigger gap and land either. Before long you will be able to make this root pass without even looking at it. Remember, you need at least 92 amps with 1/8" rod for a good penetration root.
As far as your filler and cap: HEAT is the most important issue with Low hydrogen rod. Hot on your overhead, then cool it down when you start verticle, and even cooler when you round the topside. Plenty of quick oscillation underneath to compensate for the heat. Whoever does the exray and bend test will look closely at the underside of the pipe. This is where most guys screw up on a test.
Good luck, I'm sure you'll nail it.