When Intervals was first developed as an internal tool it had "type of Task" as an attribute. For example, "bug", "feature", "improvement", etc. We found that the additional attribute for a task wasn't really necessary. Intervals assumes "a task is a task" and works well with Bug Tracking. We use Intervals to handle bugs for our service projects at Pelago. In fact, we use Intervals to manage Intervals bugs...are we allowed to admit that?
Depending on your work flow you can add a module for bugs, a project for bugs, or just treat bugs like a normal task. That is what we do. We typically attach a screenshot to the task with the steps used to create the bug and the expected outcome.
In regards to who can submit a bug, we encourage all of our team to submit bugs. The Work Request Queue can be used to submit a potential bug and then the Project Manager can review it, prioritize it, and decide whether or not to assign it.