Im not certain about summons too, they works like a spike aoe skill but damage depends on their level. As for SR eidolons leaving them at around 1-20lv is sufficient for now. I only levelled diabolos for jewels and yamato no orochi because i didnt have extra space at inventory. Its best if you dont level any SR-R eidolons and hoard them for our first event which gives us a very decent summon.