I personally don't have any experience with eco complete so I can't advise you on that. However, I can say that due to the problems regarding the edge's light restrictions (the hood cuts out light to the outer edges of the tank) I would plan on planting the HC directly under the light. I'm not sure which edge model you have but if you haven't already upgraded the halogens to MR11/MR16 LED's that's something I can't recommend enough. For HC have you looked into the Dry Start method? It might be great to get your HC carpet started
In terms of gravel/substrate, there are many options available out there. I personally use Fluval stratum with seachem root tabs and my plants are growing like crazy. I have never used eco complete so I don't know if the nutrient content in that substrate is enough for most plants.
I've read of some people growing HC successfully without the need for CO2 supplementation. Also, I would warn you that using Excel (at least for me and a few other people) hasn't worked with keeping mosses / moss balls. It slowly killed them and deteriorated them over time. If you do use excel make sure you do not overdose! Right now my fluval edge is running with some medium/high light plants without excel /co2 and they are growing like crazy. There is no right or wrong way.. At the end of the day it's all a learning experience.
To my knowledge there is no need to siphon the substrate if you have a heavily planted tank (unless you have sand/substrate that you want to keep looking pristine). It may be a good idea to occasionally lightly siphon the top of your HC carpet to remove debris and waste but I would only do this once the HC is really grown in or else you'll just rip it out of the substrate.
I strongly recommend looking through the official fluval edge thread
as well as doing a ton of research before you proceed with anything. Proper planning will save you tons of time, money and headache down the road.
I hope I could help. PM me if you have any other questions