There's no requirement for them ALL to be the same element. ANY same element Eidolon will give 10% bonus stats to all same element Soul/Hime in party. As nonsensei said, it shows up in the Edit screen.
Each Hime also has a preferred weapon type, and gets 20% extra stats from all weapons of that type. This is usually worthless for F2P players, but it's worth a giggle just how much more stats your Gun lovers (Dartagnan, Mammon) have when you have lots of Guns in your Grid.