School me on modular suppressor mounts

I have five rifles, all with threaded muzzles: my .22 boltie and my AR15 have 1/2"x28, while my .308, 6.5 and AR10 have 5/8"x24, pretty standard. Now that the NFA taxes have dropped to zero, I'm interested in getting some mufflers. I would just go direct-thread all around, were it not for the ARs; if only for appearances' sake I want to have a muzzle device on them even if the suppressor isn't, and that apparently means going modular-mount with HUB.

My understanding is that HUB threads are universal or close enough, and allow the use of 1 3/8"x24 adapters, which can also serve as muzzle devices (brakes, cages) in their own right. That, however, prompts a few questions:

• Is there a standard for max device length inside the suppressor? Sure, the threads are standardized, but that three-stage 2"-long brake or A1 prong flash hider is gonna need room in the can before the first baffle. Does the HUB standard also define these allowable dimensions to ensure any mount really does fit any can?
• Am I gonna need a shimmed/crush-washered mount on every muzzle, or will I be able to take a can off an AR's muzzle device, screw in a direct-thread adapter and hand-tighten that onto another rifle?
• is the HUB system truly standardized, or is it another situation like the AR-10 where it's really two or three similar variants?
• Any other gotchas on the topic I should be aware of?