Cleanliness is next to godliness. I have used old flux, 50/50 solder (which you can’t get) 95/5 solder etc. the only thing that really matters is getting the copper clean. The easiest way to do this is with a round wire brush (sold with plumbing junk) but I have also done it by rolling up a strip of sandpaper. Clean BOTH peices to be soldered. Apply flux. put the fittings together. Apply heat to the fitting not the solder. Don’t try to solder it until the solder flows freely when you touch it to the fitting.
Remove heat when the solder flows into the fitting. Don’t fiddle with it till it cools a little.
Nonsense. The national building code does not allow 50/50 solder in potable water system, but it is still used widely in stained glass, electronics work and other applications. It's available in all wire sizes, bars, flux cored wire, and solid core wire.