7018 isn't well suited for vertical-down root passes, due to it's heavy, sluggish, fast-freezing slag. The thick heavy slag tend to cause inclusions when trying to fill an open root gap on pipe. It's traditional to use 601/6011 for the root pass, because it performs far better for open roots, 7018 is used for the filler passes. It's also traditional to use TIG for the root which generally produces far better root quality than stick welding. the root pass is critical because the greatest stress is on the inside of the pipe. If welding is absolutely your only option the best quality will be had by TIG welding the root, instead of 6010/6011. I would strongly recommed TIG roots. It is quite difficult to get a consistent root pass on small pipes. With pipe welding quality prep work and accurate fit-up are extremely important. Furthermore for small pipes, using screw-on fittings will likely save you labor in the long run. Not to mention being probably the more reliable option. Using a pipe vice and a pipe threading tool takes less time and skill than properly done fitting and welding. You might also consider brazing with slip-on joints. Brazing is faster, easier, and generally less prone to leaks than welding.