Looks good! perhaps it would be better to put the spinach in the upstairs system, because it has a lower optimum temperature. I think it would also be nice to try and seed watercress: http://en.wikipedia.org/wiki/Watercress . Like mint it is a semi-aquatic plant and even according to wiki it grows well in aquaponics systems.
I was also thinking about Valerianella locusta (cornsalad, veldsla) It does not have to grow as big as normal lettuce. It can be harvested only 2 months after seeding and it likes colder conditions (http://www.vergetengroenten.be/index/artikel/86). It would work great for the upstairs system. The distance between plants should be 4 cm, so that would be 25*25=625 plants per m2.