Re: fertile substrate
I imagine fertilizing it would help. The concerns are with how you fertilize - if you give it what the plants actually need, if you apply it so that it doesn't get released into the water column and cause problems. Another BIG question is if it will help to the extent that you expect.
If you are fertilizing your water column and don't have any huge root feeders, I wouldn't bother with adding stuff to flourite. I have a 55 gallon with 3wpg, pressurized co2 running into an external reactor, dose EI and have a 100% flourite substrate. I've never felt or seen the need to add anything to the flourite. They seem to get all they need from the water column.
And it's nice to be able to dig around all I want without having to worry about a huge cloud of trapped fertilizer pluming up into my water column.
Now, if you just want to try it, then you certainly can. But it isn't a requirement for healthy, fast growing plants and if you do it wrong it most certainly could hurt things rather than help.
If you want to try and see, I suggest only mixing substrate fert in with part of a section of your substrate. That way, you can watch the growth in the entire tank under equal conditions and get a good idea of whether or not the substrate fert is actually helping growth or health. Doing a small section would also reduce your risk of causing problems on a large scale.
If your tank is low light, non Co2, then enriching your substrate becomes more of a benefit.