I have a few freeware photorealistic airports and areas about the airports installed. Can I install FreeMesh without removing those areas and if I do, will there be a problem?
New add-ons will usually overwright the old or default ones.
If they don't it will be a bit messy.
If you remove anything then install another they can be at different levels, that's why you will at times see the landing gear sink into the ground.
FreeMeshX works just fine with photoreal scenery - in fact, we recommend it.
In the FSX scenery library is shouldn't matter what order you have the scenery as the mesh is different entirely.
You can read our photoreal article here (freeware):