If you take metamucil or flaxseed too soon after taking medication, sweeping them through the digestive tract too quickly, is there any risk of injury?

Not as you surmise: Metamucil and flaxseed in can reduce constipation in the same way any other natural or supplemental fiber does. Neither speed up intestinal transit time to any significant degree or cause injury in recommended doses. However, fiber can bind to certain medications, decreasing their absorption. As a general rule, you should separate fiber supplements from medication intake by two + hours.