Does passe compose change in plural form? and how? (in each different one)
Why don't you read the explanations given in the Sentence Discussion forum (on a sentence by sentence basis, which is more precise) or the Tips and Notes?
Everything it there.
Passé composé changes whenever there is: a direct object, the verb is reflexive, or when it is a house verb.
If there is a direct object like in this sentence: Je te les ai envoyées hier, the modified verb should be changed to match the gender and quantity of the object. In this case, any plural, feminine noun is taking the place of "les" for the direct object, shown by the "es" added to the end of "envoyé."
If the verb is reflexive, the modified verb must agree with the gender and quantity of the subject. Don't forget that in French, a group of people is always masculine if there are one or more males in the group regardless of the size. If all constituents of a group are female, the group is feminine. Nous nous sommes lavés les mains avant de manger is an example of a group of people that is made up of one or more males and possibly some females, indicated by the "s" added to "lavé."
If it is a house verb, the same rule for reflexive verbs applies to the modified end verb. Je suis partie à la plage is an example of a female saying "I went to the beach," indicated by the "e" added to the modified verb "parti."
In other words, if the auxillary verb is "avoir," the modified verb changes based on the direct object, and when the auxillary verb is "être," the modified verb changes based on the subject.
It is also worth noting that you will never add an "s" to a past participle that ends in an "s" like "pris" unless the "e" comes before it.
Just one tiny problem with the example you chose in the reflexive verbs: nous nous sommes lavé les mains avant de manger - because you're washing the hands and not yourselves. Nous nous sommes lavés would be correct, but as soon as you add a subject behind, the verb is no longer modified.
There's also another case similar to the direct object - that is whenever the object you are referring to comes before the verb, you modify the verb: c'est la pomme que j'ai mangée, but j'ai mangé la pomme.
Just to add to your statement...
Passé composé is a verbe tense composed of two words (hince the «composé» part). the first word is called «auxiliaire» (auxillairy) and can be either the verb «être» (to be) or «avoir»(to have; more common ) always use in the present tense. the second word is called a «participe passé». This is where it gets weird.
As you stated, participe passé changes based on the auxillairy verb. With «être», participe passé are treated like an adjective: tuned with the noun or pronoun that is the subject. (ex. «je suis aimée». «Je» = «female», so «aimé» = female, therefore «aimée» takes an extra «e»).
when the auxillary is «avoir», the modification are liked to the direct object, not only in term of gender or quantity, but also, and more importantly, on placement within the sentence.
ex. «J'ai acheté [les fleurs]». («I bought the flowers») «fleurs» is female and plural, but here ,«acheté» is not modified.
ex. «[Les fleurs] que j'ai achetées» (the flowers I bought») Here, «acheté» has been modified in «achetées».
The direct object is the same, but the placement changed. This is the key point: if the direct object is placed after the verb, the paticipe passé stays the same. if the direct objet is place before the verb, il is modified.
Hope this helps!