Action Fleet did Starship Troopers and Aliens and Star Wars many years ago, their stuff is still sold in eBay and it's very nice and durable. They were sold as toys and so have a lot of play value. I got some when they hit close out in the stores. The are about the size of "large" 1/72nd scale figures.
There have been a lot of Aliens toys out there and not all of them were licensed. The general shape of the alien seems to be okay to make without a license, but I am no lawyer so don't listen to me.
But how large is an Alien? So they are around if you look for them.
The nice thing about the Action Fleet stuff is all the figures and vehicles are the same scale. You can use Starship Troopers to fight aliens or Imperial Storm Troopers.
Just do an ebay search for "Action Fleet" and they will show up.