I think the fact these guys don't have the opportunity to play in three areas like DC, VA and MD is why they play 7.0
They are simply playing the only options they have.
7.0 Mixed is clearly the lowest level of tennis these guys play.
The OP just needs to figure out what the dynamics are in his area. If it like yours, 7.0 mixed would be great for him.
If it like our area, I would suggest playing both 6.0 and 7.0 if he can't determine that beforehand.