The Rigify documentation says that
It is based around a building-blocks approach, where you build complete rigs out of smaller rig parts (e.g. arms, legs, spines, fingers...). The rig parts are currently few in number, but as more rig parts are added to Rigify it should become more and more capable of rigging a large variety of characters and creatures.
Based on this description I assume that it should be possible to create arbitrary (non-human) rigs with Rigify. However, all the Rigify tutorials I found start with the creation of the human meta-rig from the add menu, and no additional bone is added to it. I see that in Edit mode I can "add a sample" from a list of items, but I don't understand how to use it. For example how could I rig a spider with Rigify?
EDIT I found a BlenRig vs Rigify thread on blenderartists.org where everybody agreed that Rigify (as opposed to BlenRig) should be able to handle non-human characters. (But they didn't say how exactly...)
"Rigify is a general purpose template based system and it will let you create custom rigs for any character without really needing to know the mechanisms behind the rig. (...) On the other hand, BlenRig has a more limited use, cause it is (for now) intended for humanoid characters. So, any other rigging procedure would have to be made manually. In other words, if your character has 4 arms, you will have to duplicate BlenRig's arms in Edit mode manually, there is no "add arm" button."
"Rigify is intended to be/become a very general solution for creating the armature and controls of any character via a building-block/template paradigm."
EDIT 2 To clarify, my basic question is: how should I use the "Add sample" button (which appears in Edit Mode in the "Rigify Buttons" panel, see the screenshot bellow) that adds a rig building block? Am I not supposed to use it? (that would be weird). If I am supposed to use it, what are the rules for building a meta-rig? Can I build a meta-rig from scratch or should I always start with one of the two meta-rigs (Human and Pitchipoy Human)? Can I combine Human and Pitchipoy Human building blocks in one meta-rig or are these incompatible?