I know that 'ci' can mean 'us' and 'there', but it appears it can also mean 'it'. For example:
ci penso su - I'll think about it
I've heard that it means kind of "it, in general" but I find the rules for it confusing. Are there any hard and fast rules? When and where can 'ci' be used in this way, e.g.
ci leggo
ci parlo
ci dico
ci metto
Also, in the case when it can mean 'it', how could you say whether it means 'it' or 'us' in a phrase such as, for example:
ci parliamo
Ci is indeed a tricky italian word, has it has several different uses and I'm afraid there are no fast rules, as its meaning greatly dependends on the context.
I'll try to go through some of the most common uses
It can be
Il professore ci ha visto copiare.
which corresponds to
Il professore ha visto noi copiare
The professor saw us copying
Il professore ci ha detto di non copiare
which corresponds to
Il professore ha detto a noi di non copiare
The professor told us not to copy
Ci vediamo spesso
We see each other often
Ci sono stato
which corresponds to
Sono stato lì
I've been there
Ci sto proprio comodo qui
I'm really comfortable here
In some cases ci can be incorporated in a verb, some notable examples being: esserci (to be there/here), volerci (to be needed/required), and metterci (take).
Ci sono I'm here
or
C'è qualcuno? Is there anybody?
and
Ci vuole una laurea per fare quel lavoro
A degree is needed to do that job
and
Ci si mette un'ora per tornare a casa
It takes an hour to get home
And really, many many other uses, especially in combinations with verbs.
If we had to read those examples as you listed them, verbatim, the only one that makes sense is the second one: ci parlo?
The other ones don't convey any meaning. If you take a look, all of them require Direct Objects, except parlare which in Italian requires an Indirect Object (parlare a chi?).
As far as I know there are no hard rules, except for those cases that you listed, but if I find something I'll make sure to edit my answer.
