I installed 2 extra stabilizer jacks on my previous trailer (31 feet long). I tried using all 6 but somehow always had an issue and I finally quit using the 2 middle. The problem was, getting all 6 on the ground with equal pressure. It seems that 1 jack was never snugged up right. When that 6th one was in place, it would cause one of the other 5 to loosen up. After a year of that, I simply quit using all 6.
You can add extra jacks, it won't hurt anything (it might not help anything either. A little trick you may try is to put the new jacks at an angle, rather than square to the frame. And if you add more, add to the main frame of the trailer, not to the slides. The slides need to move with the trailer, like a wing on an airplane. If you tie the "wing" down and the airplane moves, it will break the wing off. A little exaggerated example, but the idea is the same for a trailer with a slide. The slide and the body of the camper should move as one unit. Support the main body and the rest will be OK. Let the "wing" hang free.
One other thing about movement? OK, when was the last time you went shopping for a new house... specifically a new house construction? Ever go to the second floor and have your kids jump up and down at one end of the house, while you are at the other end on the second floor? If not ... you should! You'll be shocked how much the floor shakes under YOUR feet when they jump up and down, not to mention the wall wabbling too!
After that experience, I realized a trailer will never be rock solid unless the wheels come off and the frame is placed on solid concrete.... then? Maybe?