In proceedings under this Rule, respondents shall be entitled to representation by counsel. A respondent who has been judicially declared mentally incompetent, or who has been judicially committed to an institution for the treatment of the mentally ill, shall be defended by his legally appointed guardian or guardian ad litem, if any; in default thereof, the Chief Justice, on certification by the Professional Responsibility Tribunal, shall appoint a guardian ad litem. The same procedure shall apply to a respondent who has asserted his incompetence or whose incompetence to defend becomes apparent during the proceedings. In all cases, counsel previously selected by the respondent will be appointed guardian ad litem, absent clear and compelling reasons.