Anything over 1/4 your recommended payload as a constant load (rack, sliders, skids, etc) and I bet you'll find it's time to look at stiffer springs. That's a slippery number and really an educated guess but ya gotta keep in mind that Toyota has been building suspensions for unloaded US street use for a while now, and while they may rate your truck with a little over 1/4T working load (max IIRC is 1400#ish, working load is half that), you'll be all over the bump stops with 700# in the truck offroad. I was all over the stops with an unloaded truck and stock suspension, now with stiffer springs and the load I carry I can still flex to the stops but my butt isn't dragging
. You can get by with stock suspension and heavy loads, but your capability offroad will suffer.
When you're adding tube, here's a quick rule of thumb. Thicker walls means the tube is less likely to deform or crush from a point load--good for sliders and other body protection. Larger diameter means stiffer tube, so you can drop the wall thickness if it's not body protection...for example a rack. You'll find there are some combinations where you can use a large diameter thinwall tube and it will be stronger as well as lighter than a narrow tube with a thick wall.
Basically, diameter has more effect on strength and stiffness than wall thickness, so if you choose the right stuff you can keep the constant load down and save space for steak and beer.
Something else to consider when weighting the truck is keep as much as you can in front of, or centered over, the rear axle.
