And the NTRP difference cannot exceed 1.0 between players. You cant have a 4.5 and a 2.5 playing 7.0 for example.
What is your record at 3.0 mens like? If your record is bad, I wouldnt play 7.0 because your 4.0 girl partner is not going to enjoy playing with you lol. Remember, as a 3.0 man playing 7.0 mixed you're going to be expected to play "even" with a 4.0 woman and not be a "liability" against a 4.0 man on a competitive team.
If you're just playing for "fun" and your team is for "fun" then go ahead and test yourself in 7.0. If that team is a competitive, going-for-it, team, I wouldnt even think about it unless you're at least 80% wins in 3.0, preferably at doubles.
To put this into perspective, you will be up against two 3.5 players who are likely 4.0's in ability once you get to the playoff level. In order for a 3.0 man, 4.0 woman, combo to work, your girl is going to need to play "at level" (meaning 4.0) and you're going to have to play up, way up, to a strong 3.5 or better to stand a chance.
4.0 is when players start to become much more "competitive" and a lot less "social".
Here, mixed is one of the most competitive leagues there are. You have "3.0's for life" here who go to nationals on a regular basis who play mixed exclusively.
My record this year in rated NTRP matches is 11 wins 4 losses, but I didn't get bumped up. I consider myself to be a strong 3.0. I want the matches to be competitive, i want to be driven to play better by my teammates. I feel like I'll be more challenged in 7.0 than 6.0.