I have had very similar experiences with working with ADHD middle and high school students in my school. I have one tip for you Quiz issue. I have been using a US textbook that comes with a test generator- ExamView. I was able to find a programmer in these Moodle.org forums that created a plugin to make ExamView work pretty good. If you had the testbanks, and could find time to put together quizzes and save them in ExamView you would only have to import them into Moodle. One more thing, I am looking at a program that runs on a Windows server (I have installed it on a PC to use this coming school year) called LAMS. I like what I have seen about LAMS. I can not wait to try building lessons for my learning challenged students.
Also- I have disabled chat. Forums seem too daunting, but I tried this strategy with my students and they liked it: I posted a photo of a famous scientist. At the time, we were studying science in class and introducing a new topic- Energy. It was Albert Einstein. Some students knew the answer immediately and could post the answer in the forum. I temporarily turned off Read Tracking so the students could not get the answer from the smart kid in the class. After everyone had time to post they guess- I turned read tracking back on. It worked much better than just letting them post and the everyone sees the answer and no one really tries to look. They were asked to rearch on the web for the persons name, but if the person is in your moodle they could find it in the lesson... Try that instead of letting them chat. You are right, they get off task FAST. Good luck!