word choice - Plural form with -たち／-ら

I found a similar.question here on stackexchange, but I can't properly understand the answers given there. And the textbook Genki doesn't give much of a detailed explanation on the plural form.

What I've got so far is たち -　the ultimate suffix which helps to make the word plural, for example 私たち or あなたたち.

I think it only works with pronouns? If so, can it be loosely translated as "and company"?

But then how can I form the plural form of other words? As far as I know it's not right to add this suffix to every existing noun to make it plural.

For example, if I want to say "pictures", would that be something like 絵画たち or 絵画ら? Or are you supposed to find a proper kanji that represents the plural form in this case?

The other way I know of doing this is to just "double" the noun like this: - やまやま.　２ｘ山 = mountains, right? But does it work for the example above?

And what about ら, eg. かれら. Can it be used for living things etc. as well?

Finally, can you use this way of turning words into a plural form in a formal/informal context?

Answer

It is not that it can't be used with nouns. It cannot be used with anything that is inanimate. Thus, since a picture is inanimate, 絵画たち would be ungrammatical. You should just use 絵画 regardless if it's just one or many. There won't be misunderstandings if the number isn't important, and if it is important, you should specify it, even if the number is general (like 数).

Also, I disagree with 達 being called a "pluralizing" suffix. X達 means something like, "the group of people that includes X", which is different from the English plural. The English plural means there are two or more of the same thing, whereas 達 doesn't imply that. "Students", for example, means there are several students, but they are all students; 「学生たち」 means "the group that includes the student", meaning there's at least one student, but not necessarily all of them. Of course, in reality, it's likely they're all students unless context hints otherwise, but it doesn't have to be that way.

Unlike English, Japanese doesn't usually care about the number of things to begin with. In English, you're always required to give a hint at the number of the item, even if it's unimportant. In Japanese, there is no such requirement, and listeners will guess from context what the number of the item is if you don't give them the number.