Just realised what you are saying here. I wouldn't try a massed start if you are new to clipless. If you have them on Sunday, wait a moment at the back for the bulk of cyclists to get moving, then set off without the crowd. It will only at 30 seconds or so to your time, and could prevent an embarrassing pile up.
Yep think I'm going to swerve it @jefmcg , 150 riders I need to feel comfortable so will stick with the current pedals & my Northwave touring shoes without clips for the event & then practise on my own next week with the clips in