I'll address the general questions:
Many times i hear great pianists employing rubato where the piece has no indication for such thing. ... Where does these liberties come from: is this a matter of interpretation (player's choice) or is there something guiding these performances, like "composer X piece should be played more freely"? And if there are guidelines, where can i learn more about it?
Yes, if "rubato" means "not playing with perfectly strict metronomic rhythm," then this is a liberty that can be afforded to the player. And in fact, often it is not explicitly called for because it is implicitly expected. The sheet music doesn't tell you the audience to clap or you to bow, but these are conventions. In fact, I would guess that any music that expects no rhythmic freedom is more rare. I remember an impassioned lecture by a piano professor titled "Please Don't Play in Time," amounting to "Look, just because the music doesn't tell you how to bend time doesn't mean you shouldn't; rather, it's a clue that you should."
But how much freedom you give yourself, and the particular ways you exercise it, should not just be based on your own whim, but on informed "performance practice." Which is just an official phrase for "How people did it [in a given time, or place, or genre]." Some composers might actually expect absolutely strict metronomic rhythm; I'm not sure Steve Reich has strong feelings on the subject, but I wouldn't attempt his phasing compositions without keeping a steady tempo. Note, the entire notion of being "metronomic" is not a given for all periods of music. I mean, the metronome wasn't even invented before a certain point, of course; but even after that, there was a societal sea-change to embrace mechanization in the 20th century, and prior to that, a "mechanical" tempo might be not only undesirable but un-conceptualized.
The amount and "style" of rubato I use would depend on what I'm playing. In the case of Chopin, there is a lot written about his playing (I think, without bothering to look up documentation, that this is the origin of the term "rubato"?). In particular, contemporaries make note of the alterations of tempo in his right hand, but how the left hand was unaffected by them. Meanwhile, a classical or baroque work might manipulate tempo or rhythm according to slightly different conventions. Another factor that matters a lot is whether you are playing alone or with other people. A baroque solo-instrument "prelude" or "fantasia" explicitly expects a quasi-improvised freeness of tempo, that is perhaps different each time (but often following the same principle as Chopin: The "big beats" are spaced regularly even if what comes between is mutated greatly). But if you play as an ensemble, and especially if your material shares its rhythm with another player, then you can't just do whatever you want on the spur of the moment. (Though, even in a large ensemble, there can be fluctuations of tempo as long as they're agreed on and expected. Some such are ingrained: a waltz band could probably play the Blue Danube, with it's gradual accelerando, without a conductor. Some are familiar rhetorical gestures, like the slight ritardando or fermata going into the "big finish" of a musical number. Similarly, some historical ensemble music might make similar fluctuations without being told, simply because it's "how it's done.")
So how do you find out how rhythm and tempo are handled in the relevant performance practice for your piece? You have to do some research. Ultimately, to be a performer, you also have to be to some degree a musicologist. The most useful kind of research might be listening to other performances, but beware; some might be, themselves, uninformed performances; others might perpetuate a "canon" of practice that is different from the composer's intent. There are certain turns of timing that are expected in Gershwin's Rhapsody in Blue, but the original 1924 recording, with Gershwin at the piano, is lacking in them (as well as taking some radically different approaches to articulation and phrasing):
It also behooves us to read whatever there is to read about our genre, both primary sources and secondary sources who argue over how to interpret the primary ones. This can be problematic; what does it mean when a pre-metronome writer says something is "free"? How free?—but it does at least give us some groundwork.