There were some Ops O's who I couldn't get in touch with but there were other Flt CC's or ADO's who I still had easy access to, so I put down their information and then let them know that they were my reference for that time period.

So I would think that you could put down a FLT CC or even one of your instructor pilots if that's all you could get contact info for.

I think you want to put down someone you STILL know if possible. You have more of a chance of them letting you know if they got called and how the conversation went.

I don't think they really care exactly what position the person held, as long as they supervised or depended on your performance and could comment on it.