So as you can see in the pic, there are a lot of unnecessary polygons on some faces, but if I use polygon redction it reducts the polygons in general and I would like to get polygon reducion only in faces, not the entire model.
Well this should be rather simple, so first select the polygons that you want to reduce, then disconnect them or however it is called to make them a separate object, delete the original ones, then put the objects you made into one and then use polygon reduction.
why do you want to reduce polys? There are not really that many there. They are not clean (looks like it was done in something other than C4D or maybe it was but it was booled) but there aren't a lot. You could select the faces and run Melt.