They get added to your assault or element damage multipliers depending on what the text says. For example, fire character atk is not the same as plain text just saying fire atk.

You will get the base stats of all your own eidolons but only your main eidolon will have its passive effect activated and the support eidolon you choose to use at the start of any battle, even if that one isn't your friends, you will still get its effect.

There is no stat limit from eidolons. There are limits and caps in the game but eidolon stats is not one of them.

You're better off using a R eidolon that boost fire atk that you can find in gem gacha or the first story area than using a SSR eidolon that doesn't support your team at all.