The answer to if the sand will sink through the eco complete is yes. I made the mistake of capping my Flourite with sand and it slowly sank into the flourite and now i'm struggling to separate it.
As for suggestions on aquascaping i would put some Jungle Val or Corkscrew Val along the back wall to make a nice background plant. I would let it grow big and drape over the top of the tank a little bit to make a nice greenish glow to the tank.
OH! Almost forgot! A banana plant or water lily would be a nice hiding spot for fish that like top cover (Like gouramis)