Use LOD
When you want to see 500 tanks on screen, that means the camera is very far away from the scene so the tank can be shown with less detail, I guess no one will notice that their wheel and tracks is moving or not. Except the 500 models occupy the same spot since you forgot to add collision
Then when the camera come closer, use the tank with more details, maybe using Superku's method, but instead of separating the wheel from the tank(and having them as different entities) you can stick them in the model and rotate them by animating it directly, you can control the rotation by using ent_animate.

[EDIT]
usng ent_setskin affect all entities sharing the same model, that mean you can't have one model animated while the others not