So here's my bike model and I'm trying to reduce the amount of polygons in multiple objects by using ProOptimizer. The problem is that every time I'm clicking calculate, then reduce the amount of vertex in % and just after I click on other object to reduce vertex %, the previous, or any other that got reduced already, reset and goes back to original polygons amount. Under the calculate button a message appears saying: "Optimization Invalid" I really need to make the model to have below or exactly 5000 Polygons!
Unfortunately there's nothing easy about this. Polygon reduction is a bit of an art, in and of itself, and the more you learn about polygon modelling the more you'll understand about reduction. This is unfortunate because most people need the polygon reduction first, from commercial models.
Some steps to understanding how to solve this particular problem:
Turn the model into a Polygon model, instead of a Mesh model
Separate the objects/parts of the bike out into individual objects so you can add modifiers to them and make changes independently.
Add the optimiser modifier to each object and experiment with reduction...
Long story short: learning how to do polygon modelling will help you realise optimisers are wildly guessing things that have little to no hope of being truly helpful. After decades of 3D tools of this sort, custom modelling by professional modellers is still an enormous part of 3D game content creation. People compromise between model fidelity/quality and polygon counts better than algorithms can. That's probably not going to change anytime soon.