There are always problems with making exceptions. Its easier to just have one rule than to say 'No costumes unless you're in the 501st Legion and can prove it.'
Costumes might not confuse CMs, but they could easily fool a child. And thats when it could get dangerous.
Well given the fact that it's the 501st Stormtroopers who actually walked around the Studios, stationed around the park, on patrols, marched in the parades, and even did the Meet and Greets at Star Wars Weekends, I'm pretty sure that they already have credentialing worked out (wait, you didn't think Disney actually made all those costumes and paid all those people, did you?).
Not only will Disney likely allow them, but I have a feeling they must already be talking about doing deals with them to get regular patrols and such. I won't be surprised at all if they actually offer them incentives/discount deals to do so. It's always going to be far cheaper - they come with their own meticulous costumes (they have screen-accurate costumes - various official entities already actually utilizes fan-referenced ones for their own designs and replications, just like they now do with the R2 units), and even if incentives are given they aren't employees that require benefits, hourly wages, etc.
It may be the case that people won't be able to just walk in the main turnstiles fully costumed and just go about their day with their families, but I don't think that will matter because every actual 501st member will likely be able to be costumed in the park all they want.