Although this scenario was rather simple and boring I think you have the potential to be a good designer. firstly I suggest you read all of the scenario design library (although remember that a lot of it was written before the Titans xpack came out) and remember to playtest your scenario properly, I had to go into the editor and change it so that I was player 1 not mother nature. (the player will be the last player you placed an object for, which in your case was nature/gaia.)
also, are you playing the game on low detail by any chance? cos if you are you need to realise the in high detail units look different and change as they get upgraded. compleatly unupgraded units look awfull.
Keep trying and I think you could become a very good designer.