(A) Any person may petition for leave to intervene in an appeal before the commission, which appeal has been initiated by another party. A petition for leave to intervene must be filed at least ten days prior to the beginning of an evidentiary hearing on the merits of an appeal, unless waived by the commission for extraordinary cause.
(B) A petitioner for leave to intervene shall incorporate in the petition a statement setting forth the interest of the petitioner and, where required, a showing of why the petitioner's interest is or may be adversely affected.
(C) Subject to compliance with paragraph (A) of this rule, the commission shall grant intervention where the petitioner:
(1) Had a statutory right to initiate an appeal in which he wishes to intervene; or
(2) Has an interest which is or may be adversely affected by the chief's action under appeal.
(D) If neither paragraph (C)(1) nor (C)(2) of this rule applies, the commission may consider the following in determining whether intervention will be permitted:
(1) The nature of the issues;
(2) The adequacy of representation of petitioner's interest which is provided by the existing parties to the proceeding; and
(3) The ability of the petitioner to present relevant evidence and argument.
(E) Any person granted leave to intervene in a proceeding may participate in such proceeding as a full party or, if desired by the intervenor or if so determined by the commission, in a capacity less than that of a full party. If an intervenor is to participate in a limited capacity, the extent and the terms of the participation shall be at the discretion of the commission.
(F) The commission may allow the filing of amicus briefs and may permit oral argument at hearing by amicus curiae. Any person wishing to participate in an appeal as an amicus curiae shall move the commission for leave to so participate. Amicus participation is discretionary and will be permitted only upon the terms and conditions imposed by the commission.