R.Regarding the idiom
If I want to emphasize how strong a particular quality of a person is, can I say:
He's trouble with caps! => In writing, I mean "He's TROUBLE!"
He's a jerk with caps! => In writing, I mean "He's a JERK!"
This is not good writing, at least in the traditional sense. You will not see a published book, for example, with capitals used for emphasis.
The way to stress a word is by the use of italics.
In writing, I mean "He's trouble!"
Are they intelligible in speech?
I've never heard of them. The idiom is "He's trouble with a capital 'T'".
(By the way, that implies 'Trouble', not 'TROUBLE').
If you said "He's trouble with caps!", I'd have to ask you what you meant.