In the case of the si impersonale there are some particularities to be considered. We have already seen that in the case of the si impersonale the si is not a reflexive pronoun at all, it is a simple indefinite pronoun like everybody, anybody, etc.. There is therefore no reason to treat it like a reflexive pronoun, but that is exactly what happens.
Let's remember. If there is a reflexive pronoun the verb is always conjugated with essere.
Beispiel
a) Lui ha lavato la macchina.
He has washed the car.
but
b) Lui si è lavato la faccia.
He has washed himself his face.
Until now nothing spectacular: In case b) si is a reflexive pronoun and reflexive pronouns are conjugated with essere. In the case of the si passivante (don' t confuse it, never, never, never with the si impersonale) we have a really reflexive pronoun, at least from a grammatical point of view and therefore the verb must be conjugated with essere.
example for si passivante
Si sono letti molti libri.
Many books have been being read.
literally: Many books read themselves.
But know let's have a look at the si impersonale. In the case of the si impersonale we don't have a reflexive pronoun, the si of the si impersonale is just an ordinary indefinite pronoun, there is no reason to conjugate with essere, but that's it what is happening because intuitively people people treat it the same way as the si passivante. Therefore, essere is used even in this case.
example for si impersonale
Si è parlato molto di questa storia.
A lot has been being spoken about that story.
Non si è capito ciò che voleva.
Nobody understood what he wanted.
Besides that the predicative noun (It is beautiful / It is a car => beautiful and car are predicative nouns) must match in gender and number with the subject IF THE VERB IS NORMALLY CONJUGATED WITH ESSERE. It doesn't matter whether the predicative pronoun is an adjective (The house is red) or a substantive (This is a tree). In this case essere is a copular verb and the predicative noun has to match in gender and number with the subject. Since the subject refers to a group (si is an indefinite pronoun and the meaning is everybody and everybody are several) the predicative pronoun has to be in PLURAL. And normally because of the fact that in that group are some men, it must be MASCULINE (we will see some exception later).
examples for si impersonale with verbs normally conjugated with essere
Si è arrivati presto.
One got there early.
Si è partiti quando si era stanchi.
One left, when one was tired.
Si era ubriachi dopo avere bevuto soltanto un bicchiere.
After one glass one was already drunken.
You could think that essere as well should be in plural, but that's not the case. In the case of the si impersonale essere is third person singular. (By the way: Don't think about Spanish, that's a completely different story. In Spanish reflexive pronouns are conjugated with haber, avere, and with avere there is no agreement between the past participle and the subject.)
Pay attention to the fact, that this agreement in number and gender between the supposed subject in plural / masculine and the predicativ noun happens only if the verb is NORMALLY conjugated with avere.
Si è arrivati. => People arrived. Si è mangiato. => People eat.
To the rule mentioned before that the predicative noun is always masculine there is an exception. If all the potential 'everybodies' are women the predicative pronoun has to be in plural / feminine.
example
Quando si è bambine si è affascinate delle bambole.
If one is a girl, one is fascinated by dolls.
We have already seen that the past participle matches in gender and number with a direct object if this one stands in front of the past participle.
example
A: Hai visto Maria?
B: Sì, l' ho vista.
A: Have you seen Maria?
B: Yes, I have seen her.
This logic is to be applied as well in the case that the verb is normally conjugated with avere.
example
Non la si è vista mai. <=> She never has been seen.
Li si è mangiati. <=> They have been eaten.
Le si è viste. <=> They have been seen.
As you can see, the topic is somehow weired. The si impersonale is not a reflexive pronoun at all, it's just an indefinite pronoun, there is no need to conjugate with essere, but that's what is to be done. That's strange enough, but it gets still more strange. If there is a verb that is normally conjugated with avere the same rules are to be applied as if there were the verb avere. The past participle has to match in gender and number with the direct object in front of the past participle.
That's definitely too much, so once again a summary of all the particularities of the si impersonale.
Si impersonale with verbs normally conjugated with avere
Si è visto. <=>One has seen.
Particularity: The si is not a reflexive pronoun at all, it is just an indefinite pronoun, the meaning is just everybody. Therefore there is no need to conjugate with essere, more logical would be (~Si ha visto~). But that' s not the way the Italians see it. (Past participle doesn' t match when the verb is normally conjugated with avere.)
Si impersonale with verbs normally conjugated with essere
Si è arrivati tardi. <=> One arrived too late.
Si è contenti. <=> One is happy.
Particularity: As in the example before the verb essere is used. But due to the fact that this verb (arrivare) is normally conjugated with essere and there is normally an agreement between the predicative noun and the subject, the predicative noun and the subject must agree in this case as well. Because si referes to a group, everybody are several people, the predicative noun must be in plural and normally masculine.
Si impersonale with a directe object in front of the past participle
We already know from the chapter before that the past participle has to match in gender and number with a direct object that stands before the past participle. This rule applies as well with the si impersonale although essere is used.
La si è vista. <=> She have been seen.
Li si è visti. <=> They (plural, masculine) have been seen.
Le si è viste. <=> They (plural, feminien) have been seen.
Particularity: If the direct object is li or le, in other words plural, you can find examples, where the verb stands in plural (~Li si sono visti~ <=> They have been seen). But this is wrong, although you can hear it. Li and le are direct objects and the conjugation of a verb depends on the subject not on the objects. This kind of sentence is wrong. Even Italians confuse sometimes the si impersonale and the si passivante. But these constructions, although they seem similar at first glance, are completely different from a grammatical point of view.
Let's see by an example that these constructions are completely different.
correct: Si guadagnano soldi.
=> can be converted in a reflexive structure: The money earn himself.
wrong: Li si sono visti.
=> can not be converted in a reflexive structure
(???). Si sees them, they don' t see them themselves.
correct: Li si è visti.
=> correct: One see them.
In the case of Si guadagnano soldi we have a si passivante, i soldi are the subject of the sentence and they earn themselves. The subject is in plural and the verb must be in plural as well. In the case of ~Li si sono visti~ we have actually a si impersonale, li ist NOT the subject of the sentence, the subject of the sentence is the indefinite pronoun si and si is singular, therefore it must be Li si è visti. Li is the direct object and a direct object doesn't rule the verb, never, never, never. If you regard li as the subject there is no direct object left.