He and 20 other people of the same mind/of the same opinion agreed to the proposal. Does that sound idiomatic?
Yes, that sounds okay.
At first I thought that it was redundant, but then I realized they could be 21 people who all agreed with each other in their united disagreement with the proposal.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.