Overall I'd say this is a good model. Your mechs are spot on, and your custom weapons for the minifigs are also a nice addition. The only thing I have to say is that the environment could use some improving. I see the little alien plants here and there, but the one thing your missing are the trees. Last time I recall seeing the movie there were trees everywhere. If you have the legos I'd say make one or two more helicopters and mobile mechs and take this a step further. Like the saying goes, go big or go home.
Ah I see. Yeah the stems are usually the best choice for when trying to simulate a grassy field, unless you want to build custom grass which is a massive headache and time and lego consuming. Besides making minifigs and large models I've been working on a side project thats going to take place in a desolate city. It's coming along nicely. I just need more large base plates to finish it.