As someone who used to clean houses professionally I've come across numerous 100% wool rugs that were somewhat challenging to keep clean without causing damage. You need to strike a balance between removing dirt and not pulling out the fibres, and the only way to find out is by carefully testing different vacuums until you find the right one. Also, weigh in the long term damage caused by grit being left in the pile of the rug. Sometimes you've got break eggs to make an omelette, and a quick, careful once over with a rotating brush roller is preferable to allowing a rug to be slowly worn away by the dirt hidden in its backing.
At the time I had about 8 or 9 different vacs on the go, precisely so I could pick and choose which ones I took with me to certain jobs. It's a real one-size-does-not-fit-all situation. The key is to start out gentle, and only step up to something more aggressive if it's absolutely safe to do so, especially if has high monetary or sentimental value.
For lightweight rugs or those with a short, dense pile, my favourite was the Oreck. The brush roller has bristles that are soft yet thickly tufted, so it will do an adequate job of sweeping without being overly aggressive. One other thing Orecks are famous for is having comparatively low suction, and again that's exactly what you need here, as it has enough oomph to carry the dirt away without actually drawing the rug into the cleaner and chewing on it.
For deep pile wool rugs my usual choice was an older Vorwerk upright. I had a VK122 fitted with an ET340 power nozzle, but any model from this series will do the same job to the same standard. Again you have plenty enough suction to do the job, but not so much that the rug gets chewed. You can also open the relief vent underneath the hood of the power nozzle if needed, which bleeds off excess suction. The brush rollers on these cleaners are of a free floating design so they don't get bogged down into the pile. Later Vorwerks (such as the VK130 onwards) have a similar brush system but are far more powerful, so you will need to set the suction at or near minimum to achieve the same effect. You know all is OK when you hear dirt and grit being pulled out of the rug, but don't find fibres in the bag afterwards.
Rugs with a very long and loose "stringy" pile seem quite fashionable these days, but they don't take kindly to being cleaned with a brush roller no matter what fibre they're made of. For these I used a straight suction cleaner, fitted with a natural bristle hard floor brush. This acts rather like a rug rake, grooming without pulling out the fibres. It is heavy going however, and you will need to make repeated passes to get the rug acceptably clean. This is why I will not have such a rug in my own home, as I've seen how much time and effort they take to look after and in all honesty I've got better things to do with my own time.
For anything especially delicate or valuable, clean as above, using straight suction only and a natural bristle floor brush if additional grooming is needed. If you don't get acceptable results using suction only, try a different floor head. They're not all created equal, and some are better than others at focusing suction into the pile. The Miele combination head in particular is a good choice, because the baseplate is designed to gently open up the pile so the suction reaches all the way down to the backing.
For durable rugs made from synthetic fibres, it's a different story altogether. Pretty much anything goes with these and you'd have to be seriously careless to cause any damage. My preferred choice in these instances was either a Hoover Turbopower or Sanitaire commercial. Set the height adjuster, push the cleaner forward in a quick, purposeful motion, and raise the nozzle up and away as you reach the edge of the rug. Simple.
Mike - to answer your question about the Hoover Junior, it will have nylon bristles.