I have to agree with Max on this. All of the units you post have too many polys to be used in the game. Set yourself a limit of no more than 1000 polys for any one unit. Of course, just over this won't hurt much (some of mine are over), but setting this limit helps.
If you're making these from scratch, try cutting down the detail in the model and use the texture to show all the little pieces. Most of the time no one will notice. If you're getting the models from somewhere else, stop using your source and look for another. It's nearly as hard to cut a polycount down from a high poly model than to just create a new one. Also, if you are getting these from elsewhere (and I hope you're not), you should at least post where they came from.
-Smitty