A 6" Newtonian will ride that mount better than an 8". However, the key point that people are trying to share is that maximum load is the maximum load, not the optimum load. If the mount is rated for 20 pounds, that might work with a 20 pound SCT or Mak set-up, though it would still be somewhat shaky. The mount may be able to handle 20 pounds, but how well will the tripod handle it? And, 20 pounds of long Newtonian is a different matter.
Carry a 14 pound bowling ball. Now carry a 14 pound wide screen TV set or a long 14 pound tube. The carry is quite different. The longer loads exert a greater torque on the mount. And if you have any sort of breeze, the long Newtonian is going to act like a sail and shake the mount.
That optical tube is 14 pounds with no eyepiece. Put a large 2" in there and and you are up around 16 pounds. Now add a telrad, about 3/4 pound. And each of those added loads will be out on the end of the tube, not over the mount. So you might choose to add 8 ounces to a full pound of balance weight at the other end.
A 6" F6 Newtonian is a LONG tube with the weight, focuser, finder, mirror, distributed well away from the mount. This gets worse as you add large eyepieces. And, as you have seen, weight is not the sole factor.
A 6" SCT is about 9 pounds and is a short, compact load that is close over the center of the mount. A 4" F7 refractor has is about 9 pounds, and has a much smaller cross section but comparable light gathering as it has no central obstruction.
Buy whatever you like. Won't impact any of us. But you opened a discussion and people are trying to help you and share experiences.
We still don't know why you want a Newtonian. In your first post you said you were only "interested in choosing the best scope for viewing moon, planets and star clusters". So, why a Newtonian?
Those are easy targets that can be handled by any type of scope. And some might suggest that, for the Moon and Planets, a refractor, Mak or SCT might be a better choice for a travel scope.
Star clusters, as a category, is all over the map so some better understanding of those would help. The Pleiades is almost 2 degrees wide. Globular clusters will often need fairly high power and you will definitely want high power for the Moon and planets.
Personally, for travel, I would prefer a 4" F7 ED refractor or 6" SCT over a 6" Newtonian . You can even take it on the plane as carry on or put it, padded, in your luggage. These can be put in a back pack for hiking. They cost more to buy, but it would travel better and have similar light gathering.
Edited by aeajr, 22 February 2020 - 11:58 AM.