Question

When co-opting new councillors in, do those new councillors immediately join and thus vote on any remaining candidates?

Answer 1

I would say no as cooption of all seats is normally a single agenda item which would result in the new cllrs signing their forms at once.

Answer 2

This will entirely depend upon your co-option policy and procedures.  If candidates have been served with the appropriate meeting notice and on the day have signed a Declaration of Acceptance of Office, they can take part in the meeting immediately after being voted onto the council. Some councils leave any co-option to the end of the meeting and some have a policy that states that the co-opted individual takes office at the next meeting so in either case they would not be able to vote on other candidates.