My friend keeps his peacock with his bala sharks, but you can't keep more than one adult bala shark in a 125, from what he's told me. But don't take my word on that.
Just be wary, while they're young the eels are really shy, and you'll almost never see him until a few months later when he gets comfy, after that you can get really close to them. Pet them, hand feed them, they're such a joy.
I think any fish marked as "peaceful" will do. Nothing that's nippy. Nothing that's too territorial. I have rainbowfish stuck in my head something serious because that's what I'm adding to my 125, but I'm sure that would work, too. I'm not very familiar about other types of medium-sized fish, haven't come across it other than my own case.
Following the thread, I'm interested to see what people will come up with.