Hello and welcome to the Flow forum!
@Eva has already given you some good advice. I agree with her - lifting one frame out at a time makes moving the brood box much easier, but if you are pretty strong, or have help, you can do it the way you describe too.
I recommend that you put 10 frames in that brood box. You may get a lot of cross comb if there is extra space between the frames, and that can be very hard to fix later. You don’t want cross comb when you are inspecting, and you will need to do that regularly throughout the nectar flow season.
As far as adding the super goes, I would only add it when all of those frames have fully drawn comb and are 80% full of brood, pollen or honey. If you don’t, the bees may have too much space to defend. Having said that, your photo suggests that you have plenty of bees, so it will be a judgment call for you to make. If you do put the Flow super on, I would suggest smearing a bit of burr comb onto the Flow frame faces to encourage the bees to accept the super more rapidly.
The way I deal with this is to trim the comb if needed, then hold it into the frame the right way and as straight as possible with rubber bands. Much safer than balancing it and hoping it will get fixed. 
Here is a photo of the concept:
I presume you mean 7 plastic Flow frames? That hive size is traditionally referred to as a 10 frame size, because that is how many wood frames you are supposed to put in the box. Semantics, I know, but it just helps to make sure that we are all talking about the same thing. 