FE is used as an 'immediately available' plant food, it doesn't need to be broken down to be or become available to the plant. If you have microbes alive and well in the media however, they're going to eat, and they're going to shit, so it's going to be passed through them whether you've planned on it or not. If, however, your medium is not 'alive' it's still not to worry. Just remember that it is indeed quite possible to burn with fish emulsion.
I do not like crab shell meal (or oyster shell) in this application because of the chemistry of the shell itself, a good bit of CaCO3 and it's specifically the CO3 that's a problem. I prefer something like gypsum in a case like this, or a liquid Ca product such as BioLink 6% Ca (which is organic) in combination with Epsom salt (aka MgSO4, and that SO4 is what helps the plant make itself stinky). I like gypsum because it's relatively soluble but does not affect pH, and it's cheap.
Now, if your source water has low to no carbonate hardness, then yes, you may want to buffer but my preferred method is to do it via the feed/water, not the medium. If you buffer the media too high then you're in a bit of a pickle.
If growing indoors, coir is superior to soil IMO because of how quickly things can happen (good or bad). Soil is a slow process that's expensive to do inside, depending on your power rates of course.
Will you see typical growth rates using organic? It depends on how you approach it. If you're going to want to go the microbial route and use products that basically must be passed through the microbe, then it's going to be a little bit slower going than if you're using salt-based fertilizers. But using things like the fish emulsion, liquid guano, liquid seaweed, etc, you'll be able to see things moving along much like classic hydro, again in my opinion. Others may have different experiences and opinions, of course.