Customizing messaging for two different groups

Hi everyone,
We are going to be administering a 360 for two groups of individuals. For various reasons, we would like to run this out of one assessment.
However, we need to customize our email messages for each group. My plan is to bring in group assignment as metadata, and then filter email recipients based on that info. I have a couple of questions:
1) Is email content fixed once a mailing is scheduled? So, for example, could I schedule an invitation for Group 1, and then before it goes out, replace the email contents and schedule the invitation for Group 2?
2) It appears there is no way to apply this to the reminders, without sending reminders to those who may not need it. That is, I can't select people who are BOTH unfinished respondents AND members of Group 1. Any ideas to address that?
Thanks!