Questions should be directed to all candidates rather than to a specific candidate, and questions should be concise and to-the-point (not speeches). Beyond that there is no restriction on questions of which I’m aware.
My personal recommendation is to keep questions as short as possible so that you get an answer to the question you ask rather than general remarks on the topic. Although sometimes candidates will be right not to answer question that are too specific - for example a specific undecided planning application. In this case it may be better to ask questions more generally about their view on planning issues - perhaps with reference to a specific development - by way of an example.