We are unable to provide a weight capacity as it would depend more on the strength of the vertical rods and how they are installed.
Unfortunately, the smallest size coupling nut we carry is the 1/4in - 20 tpi size.
It will work horizontally for the mentioned application.
Get a turnbuckle with the thread size you need. It is longer than a coupling nut, but it will do what you want.
.88" Thats the specified length.