Well, from Shanks' point of view, isn't Buggy and Mihawk teaming up kind of like two of his old friends hanging out? He probably thinks it's kind of sweet that his old childhood friend is hanging out with his old sparring buddy.
Shanks might be more concerned about them both hanging out with Crocodile, since that guy's a bad influence.
But, the thing is, I think Shanks is less concerned about "threats to his power" and more concerned with "are these people with power good people?" Because we've seen that Shanks does go out of his way to protect the weak. He seems cares about what happens to those who get caught in the crossfire between different superpowers. Shanks probably wants to keep a modicum of peace and order, even in a world full of pirates.
And Shanks knows, deep down, Buggy and Mihawk aren't bad guys. Buggy talks big, and he's selfish, but he's also willing to help out people as long as they stroke his ego. Mihawk's one of the strongest guys around, but he'd rather spend a night in with a good book than go out terrorizing small towns.
And even Crocodile, at the end of the day, is more of a mob boss or a businessman than a anarchist. He doesn't want to burn down cities and kill random people for fun. There's no profit in that for him. Crocodile might not be a force for good, but he's still a force for ORDER, at least to a certain extent.
Shanks seems like he's more concerned with the kind of people who would burn down a entire island just for funsies. The kind of pirates who would make life unlivable for the goat-herds and farmers of the world. And that describes Blackbeard moreso than anyone in Cross Guild.
I think, for Shanks, it's less about pure strength, and more about what those people actually do with their strength.
Plus...Blackbeard's just WEIRD, guys. No one can get a read on him, or what he's all about. And that probably spooks Shanks more than a bunch of guys who he does seem to know and understand.