I don't think the length of the 5er is any concern. I towed a 25' Sprinter fifth wheel trailer plus a 4,000 pound 10' box trailer for years and thousands of miles with no problems.
The first thing I learned is that when the 5er does a wiggle, the tag trailer does a big wag or sway. Not good, unless controlled by a sway bar or some sort of anti-sway equipment. I installed an Equal-I-Zer friction anti-sway bar on the tag trailer's hitch and that fixed the problem.
The second thing is to load the tag trailer so you have at least 10 percent hitch weight. Less hitch weight will result in a huge sway problem.
The third thing is that the additional junk iron welded to the rear frame of the 5er trailer to support the receiver has to be over-engineered. You may need to add an additional steel I-beam to the frame from the axle to the hitch. Smaller 5ers are not engineered to add a rear receiver, so you need a good welding shop that knows what it takes to drag a 4,000-pound trailer without tearing up the 5er's frame.
And I suspect you are way low in your estimate of the weight of the 5er. Mine had a GVWR of 7,900 pounds, and that's about how much it weighed when on the road. I often had 6,500 pounds trailer axle weight plus about 1,500 pounds hitch weight - not counting the tag trailer.
Granted, if you go with a low-profile 21' 5er you'll have a GVWR closer to 6,000 pounds. But those baby trailers usually have almost no supporting frame behind the rear axle to which you can add a receiver hitch.