#10 caps fit ruger nipples best, at least they do on mine. The manual used to say #10 (mine does... circa 1980-ish), but now it says #10 or #11. There is some variation in caps from manufacturer to manufacturer, so you may want to play a bit to see what fits best (snug, not so tight you need to push hard, not so loose they fall off).
.457 ball or "Lee old army conical".
Any amount of any type of black powder you can get in it is OK with Ruger.
Cover the balls with grease or crisco to prevent chain firing, and d/l the manual from Ruger's web site to make double sure of everything.