Eco complete is a good substrate but it lacks nutrients that plants need. It will hold the nutrients once there added from fish, root tabs etc. It wont disintegrate over time, and I dont believe it effects your ph level. I've had alot of success with eco complete, with the addition of root tabs, and osmocote.
My opinion is that root tabs lost longer then osmocote. I have also seen a better, and faster reaction with osmocote then root tabs.
Also you should rinse eco complete as it will color your water. I just poke holes on the bottom of the bag, and rinse with water a few times. Then put it in the tank.
Corys will do ok with a hard substrate, but it would be better to use sand. Could damage there whiskers or whatever there called. I have 7 green corys in a 40b with eco complete as substrate. Im growing oval ludwigia, cryptocorye undulata, pennywort, corkscrew vals, and some other random plant.
Im also in the process of setting up a 20long. Im curious to see how yours progresses. Have you decided on a light fixture? I cant decide whether to use a dual 24in T5HO thats extendable to 30in. Or a 30in T5HO dual.