following that logic , it's better to start with no SSR kamihime or eidolon at all , since you will have more room to grow.
That's willfully missing the point. A team with a +100% eidolon can clear content with weaker kamihime, and benefits more from stronger kamihime. What would be side-grades to a team that started with a fuller roster would be upgrades to a team that didn't. Beyond that we'd have to get into specifics about particular elements, and the likelihood of being able to throw together a balanced team in X amount of time with or without spending money. Unless someone else wants to chime in and lecture on that topic, I'll pass. I'll never have a +100% eidolon, so the topic really isn't that important to me. I give two options for rerollers: core kamihime, or +100% eidolons. It's up to them to choose what they want.

What about limited Kamihime?
I am building a Dark Team and my core hime is at the moment Dark Ama...
Later we will get a 100% Dark attack eidolon... So, how high is the probability to get Dark Ama afterwards? I can't buy her with a miracle ticket. If I am unlucky, then maybe I won't get her with jewels alone and have to invest a lot of money.
Dark Amaterasu is the best you can hope for as a starter kamihime for a Dark team right now. There's no point in agonizing about possibilities that are closed to you. As for chances of rolling limited kamihime, I give you the following table.