Second Sight characters, from Chargen or at the time of their Becomings, that are focused on psychic ability receive a defining merit similar to Thaumaturges. However, since psi-merits will go up to 5, psychics simply pick a particular power and gain four dots in it. Of all psychic merits (as with the thaumaturgic merits), the defining merit must be chosen first. That means it must be a 'base merit', one which does not have any psychic prerequisites. Thus, a telepathic character could choose Mind Reading at 4 for their defining merit, but could not choose Thought Projection, since that has Mind Reading as a prerequisite. The sequence of spending is very clearly defined: you must choose your defining merit first. Then you may spend your merit dots from chargen. Only when all of those are spent can you spend your starting xp.
If a power is chosen as a defining merit that requires 5 dots (e.g. Pyrokinesis), the fifth dot must be bought with two merit dots. If a power is chosen as a defining merit that can only go up to 3 dots, the fourth dot may become the first dot in another psychic merit, but again it must be spent before merit dots.
This also means that psychics have a kind of 'psychic path.' A psychic may be, depending what their first, defining, psychic ability was, a Mediumist, a Kineticist or have ESP. Or, even more specifically, they may be a Channeler, or a Telekinetic, or a Clairvoyant, respectively, depending on which merits they've selected. This 'path' is a matter of rp flavor, something which should make sense given a character's background. In most cases, the ability a character develops first will shape their sense of who they are, and will influence what further abilities they develop over time.
However, there are no rules limiting the availability of psychic merits, except for the prerequisites. As long as it made sense for the character's IC development, a character could conceivably have Psychometry (an ESP merit), Automatic Writing (a mediumistic merit), and Animal Empathy (a telepathic merit), although the explanation for how these apparently disparate talents all make sense for a single character would be need to be well-thought out and would probably be very interesting. The point is, this is something a player can work out with staff; there is no actual rule against it.