Should we survey contract or term employees?

It depends on the type of employees, some contract or term are an essential part of the organization, so YES if essential, these employees should be surveyed. However, there are other types of contract or term (like from IBM or Cisco for example or any other long term contract relationship) that associate with their firm and know the firm they are contracted with - so in that case NO.

Regarding employment length - anyone one that has been with the organization for 3 months or more would make sense.