I would also wait. Paitence is a virtue in this hobby. If you don't have paitence this might not be your thing....
That being said if you REALLY want to get the tank started you could do that, then when the Eco comes just take all the water out of the tank, and then add substrate and either put the same water or different water back in.
I had water and fish in my tank when I added my Eco. The only thing that I should have done was turn off the filter. I had to clean it every day for a week to get it start back up from the sand that would stop the impellar. The tank was cloudy for a day but other then that is went quite well.
You can also run the filter on a different tank till you get your Eco. Then it will have the Bacteria needed to have a quick cycle.
I added my Eco Complete after my tank was full. Just make sure its not too full that it will overflow when u add it. I rinsed off the bag then put in in the tank on the bottom. Then I used a knife to open the bag and poured it out. This worked really well... no floating substrate problems for me.
If I were you, I'd start to cycle the tank now, and then when the Eco-Complete comes in, drain the water into a bucket. Add the Eco-Complete, add enough water to get to ~5 gallons, do a small WC to get rid of debris, then re-add the old tank water.
Run your filter with a bag of carbon just to clear up the water some. It'll last 2-3 weeks. After that, don't use any more carbon.