Orochimaru: "Unable to enter Sage Mode because of the fragility of his host body, Orochimaru instead developed an alternate method of harnessing his senjutsu chakra through cursed seals, which he tested on Anko and several others before eventually using the finalised versions on Kimimaro and the Sound Four"
"During his research on Jūgo's clan, Orochimaru eventually tracked down the source of their power when he discovered the Ryūchi Cave and senjutsu. Here, he attempted to learn senjutsu as well as Sage Mode. However he was unable to use this power to its full potential due to being unable to find a host body that could bear such an ability, making it an unsuitable option for battle altogether."
"Combining this chakra with the enzyme produced by Jūgo, he was able to create these unique seals, which triggered a change near-identical to that of the Sage Transformation, but with different benefits and drawbacks."
Jugo-"Like other members of his clan, Jūgo possesses the ability known as Sage Transformation which allows him to use the natural energy he absorbs to transform his body at will"
These are from the NarutoWiki. I hope that clears things up. Although, IMO just knowing about the sage techniques doesn't make you a sage, epsecially if you cannot pull of the techniques. I mean, I know how Tiger Wood swings his golf club but I'm no pro golfer. As for Jugo, I always saw his sage transformation the result of his kekkei genkai mixed with the curse seal.
I personally don't think we've seen enough of the sages and think they're one of the coolest types of charters in Naruto.
P.S. You forgot to add the Sage of Six Paths to your list. I'm pretty sure he was a sage of some number of paths lol