What is the right polygon count? Firstly, I was surprised I did not find a thread like that here(I hope I did not miss it.)

So what is the right number of polygon count or better triangle count, since this is what BGE renders from what I've read. 


Is there a max tris count number that I should not exceed?
Is there a default tris count number for
small(ornaments)/medium(furnitures)/big(houses/terrains) that I
should be aware of? Experienced BGE users should propably have an
idea. Or do I keep checking till the fps drops and try to dissolve
meshes afterwards?
Objects in different layers or stand-by scenes that might appear
briefly or not appear at all in the game affect the fps? For example,
meshes in different layers that replace other meshes through
actuators or meshes in overlay scenes that get added briefly and are
removed/suspended afterwards.
And at last. Objects with dynamic physics or objects with animations
or characters with armatures or in general objects that interact with
one way or another, do they need lower poly count than a similar one
that would not act like the above and let's say would remain static
and act as a decorative level asset?


I really think there is a connection in the 4 above question-points. If you doubt it I can split them to more questions. My search so far has shown that there might not be a definite answer but more experienced users can judge better.
Is there a max tris count number that I should not exceed?


Nothing you need to worry about it. I'm sure nobody managed to get that many vertices that hit that limit and can still use Blender. Remember you have limited memory ;).


  Is there a default tris count number for small(ornaments)/medium(furnitures)/big(houses/terrains) that I should be aware of? 
  No
  
  ... do I keep checking till the fps drops and try to dissolve meshes afterwards?


It is the purpose of profiling to identify bottlenecks. It depends how much effort you want to investigate in it.


  Objects in different layers or stand-by scenes that might appear briefly or not appear at all in the game affect the fps?


Not loaded scenes just cost disk space.

Inactive objects (residing in hidden layers) just cost memory and load time, but no processing time while the game is running.

Added Objects need additional processing time to get added (which just a little). After that they are treated as normal objects.


  Objects with dynamic physics or objects with animations or characters with armatures or in general objects that interact with one way or another, do they need lower poly count than a similar one that would not act like the above and let's say would remain static and act as a decorative level asset?


Every detail causes costs in one way or the other. If you do not need a detail it is more efficient to skip that detail.

Example: A tree can have a lot of polygonal details for rendering but much less regarding physics. Other details are better presented in a texture rather then lots of polygons.

In general it is better to care such things when you have performance problems. Otherwise you spend time on aspects that might not be relevant to your situation.

Surely this is different when working on large projects, with several team members, strict budget and deadlines. Even than you will hit performance issues.