As for refill technique that's exactly what I've been doing....rotating as I drop about 5 drops, let it soak in, and rinse/repeat until I get to 20 drops.
That's exactly how I do it too. I tilt the carto about 45 degrees, then apply one drop, then rotate maybe 30 degrees around the long axis, another drop, etc. Five drops, then wait maybe 30 seconds, then repeat the process three more times for a total of 20 drops. This is for cartos that have been filled before, though.
The first time you fill a blank carto, use the same procedure as above, but do a fifth cycle for a total of 25 drops. I tried putting 30 in a new carto, but it ended up leaking through the bottom, onto the threads, and into one of my G6 auto batteries, which aren't sealed like the manual ones. Almost killed the battery, but I banged it threads-down on a table top three times, and it was working again.
So I'd recommend 25 drops in a new blank carto, then 20 each time you refill. Be careful to make sure the carto is close to empty, though, because if it's still, say, 20% full, it'll overfill and flood the threads. The way to tell is you'll detect a slight "burnt" taste when it's empty enough to refill.
And like Grimwald says, how many times you can reasonably refill each carto will vary depending on the type of juice you're using - darker juices wear out cartos quicker than light-colored ones. I found that with my light-colored all day vape juice, I could refill my cartos about 5 times. More than that, and it'd taste burnt after only a few hits, which wastes the remaining juice in the carto, as I don't know of a feasible way to "de-juice" a cartomizer.
Halo G6 cartos are metal wrapped with an adhesive paper on the outside, so you should be able to use any juice from any vendor in a G6 carto. There may be exceptions to that I'm not aware of, though, so I'd defer to wiser vapers than I.
P.S. - Some people may recommend the "condom method" (you can find it described in these forums, or you could Google it), but it's messy, and wastes the juice that you have to wipe off the bottom/threads of the carto and from the inside of the "condom" when you're done. In my opinion, the drip method is better. But again, there are many wiser vapers here who may know better than I.
EDIT: Doh! JaxMike just referenced the condom method, in fact. Some people prefer it, while some prefer to drip. Like so many things in this world, it's a matter of personal preference...