Well, having your general comments for everyone to keep a note of, instead of repeating it to each one in PM or on his own designer page, has its advantages.
Therefore, state your general shared requirements, guidelines, direction of how you'd want your logo to be heading on the design direction part.
You can use PM, if you felt like it, only if you are asking for specific changes that concerns that designer unique design you are approaching.
It is not good or recommended to ask other designers to carry on a similar unique idea represented by one designer at your contest.
Thanks,
Kareem