.223 inside unsuppressed is going to have an incredible amount of blast any way you cut it, so like others I'd tend to go shorter... 16" for non-NFA. Personally I would probably not go for one of these 7.5 "pistols"; if for no other reason than the flash they make is impressive in broad daylight, never mind the black of night with your eyes adjusted. At the least I'd shoot it some in the dark first to know what I was getting into.
I wouldn't be worried about whether a couple hundred FPS is going to lead to picture perfect fragmenation or not either.... who cares? There are 29 more where that one came from.
My first/only suppressor (wanted one for a while, just never got around to it until recently) is pending now, and when it comes in I am probably going to put it on an 11.5 AR and have that as the proverbial "bedside gun".
-I live in SC; at LEAST 25% of any given jury is likely to be highly pro-gun.
-We have a pretty air tight castle doctrine in SC... a shooting in your home of a true intruder has virtually no chance of going to trial here.
-And most importantly, I like my hearing and plan to hold on to it as long as possible. Taking the flash down to zero wouldn't hurt either.