For any project, if you carry a consultant, you need to mark up at least 10% to 20% of the consultant's fee. There are administrative cost and risk to carry any consultants so make sure you get compensated.
If your clients require you to coodinate with their consultants. You need to charged them a flat or hourly fee to deal with them . Personaly I will charge by hour or a flat fee for not to exceed a certain # of meeting.