K&F Concept camera tripod you are asking, yes, you Canon rebel sl2 could be used on our tripod. Your camera could be used as long as it has a 1/4" standard screw interface; If your lens is large or telephoto zoom lens, please ensure to expand the leg angle, and hang some weight to the hook to increase its stability. For camera with telephoto zoom lens, you need to buy a tripod mount ring additionally for better balance.The center column also has a hook to hang weight on to lower the center of gravity on this tripod to make it more sturdy and less prone to vibrations which is awesome for photography where you take long exposures . 

No, the rings are intended to adapt filters and other light weight accessories on the camera lens. 

No, The filer ring sets in general are for the weight of a filter and not intended to support a lens. Also you may alter the focal length with ring adaptors used in this way. T rings are common for adapting a off brand lens to a camera. 

That may be caused by the fact that the lens weight is much bigger than camera, If used your telephoto lens with our adapter ring, we suggest to use with a lens bracket and better to shoot on a tripod to increase its stability.