For all you guys in free states:
Here's the OP's problem: In Connecticut, it's perfectly legal to purchase and use a suppressor. However, CT still uses the "feature count" to define "assault weapons", and one of those features is having a threaded muzzle OR a flash hider (pinned and welded compensators are okay).
As everyone who's used a suppressor knows, not only does it make the gun quieter, but it also SUPPRESSES FLASH better than anything else. And this, in CT, is a very gray legal area. Sure, it's a sound suppressor. But is it also a flash hider? The Attorney General hasn't ruled on this, and neither have the courts. Oddly enough, people aren't lining up to be the test case.
So the question isn't really about the legality of the suppressor, it's about whether putting one on an AR turns the AR into an illegal assault weapon.
OP, this is one reason why preban ARs go for a premium here. Get one of them, and you can do whatever you want to.
Okay, let's forget the law (or lack of it) and confine the discussion to the multi-caliber question. Yes, a larger diameter suppressor will work, and generally work well. My longer term goal is to use the same suppressor for my Mini-30, my AR, and my 7mm bolt rifle, and all my research and conversations with people who know say it works okay, depending on the can itself, and the amount of freebore in the over-caliber can.