I know I have read before that a quizmaker should only submit one quiz at a time. But I've been hunting around the site and can't seem to find this written anywhere. I was wondering if this is still the case or if this had been changed. I submitted a quiz in a category which is a bit slow at the moment and I have another quiz ready to submit into a category which is moving quite fast. Is it OK to submit the second quiz?

Nope, you can only submit one quiz at a time. The exception are quizzes that have been online before and were taken offline because of correction notes. You can resubmit those regardless of other quizzes in the queue.