Hey, welcome to the forums
The fluval substrate is awesome. Depending on what plants you plan on growing, some fertilizer tabs/capsules might be really beneficial.
Keep in mind that the Edge has a major flaw for growing aquatic plants: getting light to reach the corners and edges. With this in mind, make sure you plant your higher-light demanding plants closer to the middle. There are a ton of modifications worth looking into for the fluval edge. There is also a list of low-light plants available on the low tech forum which could help you choose the right plants for the look you're going for.
I think your safest bet is to go with low-medium light plants for the fluval edge.
I've never kept GBR but I don't think I would put 2 in a 6 gallon. I don't believe the inch per gallon rule should apply when it comes to nano tanks. A Convict Cichlid can reach 4 inches, that doesn't mean you should put it in a 6 gallon. Just my opinion
Unfortunately one of the downsides of nano tanks is the stocking limitations.
There is a nano-fish list available on the nano planted tank forum if you're looking for nano compatible fish. I've found it very useful for myself.
Good luck and enjoy the fluval edge!