Congratulations on your scrap lead deal!
Prior to melting it down, I would suggest you remove as much dirt and scale as you can. It will make for much less skimming work. Just thump those old pipes on a hard surface, tap the outsides with a ballpeen hammer, or rod them out some way to get out all the crud you can.
Don't be concerned about wether or not the lead is soft enough. Flashing and plumbing pipes (and cable sheathing, if you can find any of it) was typically made out of pure, or nearly pure lead. I have shot lots of balls made out of such salvaged lead and it was all as soft as you please. You can depend on the ol' fingernail scratch test as a good indicator.
Good luck.