Callihan absolutely can work in this but he just won't be taken seriously on a top level against Mox. Their past, at this stage, is irrelevant. It's a factoid Excalibur would say in passing during a TV match, nothing more. His spot in Impact is just not the same as it would be in AEW, and as a result hurt him, Impact, and Mox. I think he and Darby would be a much better fit for a myriad of reasons. But that's as high as he could be realistically. If I'm wrong, I'm cool with it, but he's just not the guy as a singles act to be that high for this story. He gets over, that's good. Don't confuse me saying he's not the same league as saying he's useless.
Sting's got room to be interesting. I'm curious how they make it work, which is a first for every party involved in ten years. But yeah, dude can't bump and has no purpose beyond name value. I'd much, much, much rather they let the older guys they actively use already play to their own strengths to make guys over, but it's Sting.