Okay, let's go through these image by image.
First image: Cut down those rib thingys to simple boxes. You'll never see that much detail. If you want them still to have a rounded appearance, then bevel them and assign them the same smooth group.
Second Image: Most of those details, being so small, should be on the normal map, not on the model. I'd say redo the engine pods as one big box, and model only the largest of details. In addition, the rounding on the front should be accomplished with a single ring of faces, once again a bevel. Don't try to round objects for games that are so small.
Third image: That tube has way too many sides. It's so small I can't even see where it is on the long view of the model. It should probably be deleted entirely.
Fourth image: Get rid of that bump entirely. Handle that in the mapping. Also, a gun barrel that small should have 8 sides at most, and be a simple cylinder. Handle the changes in shape via normal maps.
All in all, your modeling will improve if you step back and look at the larger picture. A good technique I use to see if I'm using too many faces on an object is to view the entire model in one viewport. If I see two edges so close together that they look like one, then they probably should be one.
I'd suggest starting by building the entire model, and then detailing, largest details down to the smallest, that way you'll not need to exceed your poly budget.
Look at this image, and hopefully you'll see a good idea of what sort of level of detail can go into mapping.
